SDH
"SDH" is an American term the DVD industry introduced. It is an acronym for "Subtitles for the deaf and hard-of-hearing", and refers to regular subtitles in the original language where important non-dialog information has been added, as well as speaker identification, useful when the viewer cannot otherwise visually tell who is saying what.

The only significant difference for the user between "SDH" subtitles and "closed captions" is their appearance: SDH subtitles usually are displayed with the same proportional font used for the translation subtitles on the DVD; however, closed captions are displayed as white text on a black band, which blocks a large portion of the view. Closed captioning is falling out of favor as many users have no difficulty reading SDH subtitles, which are text with contrast outline. In addition, DVD subtitles can specify many colors, on the same character: primary, outline, shadow, and background. This allows subtitlers to display subtitles on a usually translucent band for easier reading; however, this is rare, since most subtitles use an outline and shadow instead, in order to block a smaller portion of the picture. Closed captions may still supersede DVD subtitles, since many SDH subtitles present all of the text centered, while closed captions usually specify position on the screen: centered, left align, right align, top, etc. This is very helpful for speaker identification and overlapping conversation. Some SDH subtitles (such as the subtitles of newer Universal Studios DVDs/Blu-ray Discs) do have positioning, but it is not as common.

DVDs for the U.S. market now sometimes have three forms of English subtitles: SDH subtitles; English subtitles, helpful for viewers who are Hearing and whose first language may not be English (although they are usually an exact transcript and not edited into Simple English); and closed caption data that is decoded by the end-user's closed caption decoder. Most anime releases in the U.S. only include as subtitles translations of the original material; therefore, SDH subtitles of English dubs ("dubtitles") are uncommon. [9][10]
High-definition disc media (HD DVD, Blu-ray Disc) uses SDH subtitles as the sole method because technical specifications do not require HD to support line 21 closed captions. Some Blu-ray Discs, however, are said to carry a closed caption stream that only displays through standard-definition connections. Many HDTVs allow the end–user to customize the captions, including the ability to remove the black band.

Subtitles in the same language on the same production can be in different categories:

Narrative This is the most common type of subtitle. Narrative subtitles are those in which spoken dialogue is displayed. These are most commonly used to translate a film with one spoken language and the text of a second language.

Forced These are common on movies. Forced subtitles only provide subtitles when the characters speak a foreign or alien language, or a sign, flag, or other text in a scene is not translated in the localization and dubbing process. In some cases, foreign dialogue may be left untranslated if the movie is meant to be seen from the point of view of a particular character who does not speak the language in question.

Content Content subtitles are a North American Secondary Industry (non-Hollywood, often low-budget) staple. They add content dictation that is missing from filmed action or dialogue. Due to the general low budget allowances in such films, it is often more feasible to add the overlay subtitles to fill in information. They appear most commonly seen on America's Maverick films as forced subtitles, and on Canada's MapleLeaf films as optional subtitles.

Content subtitles also appear in the beginning of some higher-budget films (e.g. Star Wars) or at the end of a film (e.g. Gods and Generals).
Titles only Dubbed programs use this sort of subtitle. Titles only provide only the text for any untranslated on-screen text. They are most commonly forced (see above).

Bonus Bonus subtitles are an additional set of text blurbs that are added to DVDs. They are similar to Blu-ray Discs' in-movie content or to the "info nuggets" in VH1 Pop-up Video. Often shown in popup or balloon form, they point out humorous blunders in the filming or background/behind-the-scenes information to what is appearing on screen.

Localized Localized subtitles are a separate subtitle track that uses expanded references (i.e. "The sake [a Japanese Wine] was excellent as was the Wasabi") or can replace the standardized subtitle track with a localized form replacing references to local custom (i.e. from above, "The wine was excellent as was the spicy dip").

Extended/Expanded Extended subtitles combine the standard subtitle track with the localization subtitle track. Originally found only on Celestial DVDs in the early 2000s, the format has expanded to many export-intended releases from China, Japan, India and Taiwan. The term "Expanded Subtitle" is owned by Celestial, with Extended being used by other companies.

3D 3D subtitles combine the standard subtitle position along the X and Y axis of the picture, with a third position along the Z-axis. This third positioning allows the subtitle to "float" in front of the 3D image. This option is available in Digital Cinema and in 3D Blu-ray releases.

Subtitles exist in two forms; open subtitles are 'open to all' and cannot be turned off by the viewer; closed subtitles are designed for a certain group of viewers, and can usually be turned on/off or selected by the viewer - examples being teletext pages, US Closed captions (608/708), DVB Bitmap subtitles, DVD/Blu-ray subtitles.

While distributing content, subtitles can appear in one of 3 types:

Hard (also known as hardsubs or open subtitles). The subtitle text is irreversibly merged in original video frames, and so no special equipment or software is required for playback. Hence, very complex transition effects and animation can be implemented, such as karaoke song lyrics using various colors, fonts, sizes, animation (like a bouncing ball) etc. to follow the lyrics. However, these subtitles cannot be turned off unless the original video is also included in the distribution as they are now part of the original frame, and thus it is impossible to have several variants of subtitling, such as in multiple languages.

Prerendered (also known as closed) subtitles are separate video frames that are overlaid on the original video stream while playing. Prerendered subtitles are used on DVD and Blu-ray (though they are contained in the same file as the video stream). It is possible to turn them off or have multiple language subtitles and switch among them, but the player has to support such subtitles to display them. Also, subtitles are usually encoded as images with minimal bitrate and number of colors; they usually lack anti-aliased font rasterization. Also, changing such subtitles is hard, but special OCR software, such as SubRip exists to convert such subtitles to "soft" ones.

Soft (also known as softsubs or closed subtitles) are separate instructions, usually a specially marked up text with time stamps to be displayed during playback. It requires player support and, moreover, there are multiple incompatible (but usually reciprocally convertible) subtitle file formats. Softsubs are relatively easy to create and change, and thus are frequently used for fansubs. Text rendering quality can vary depending on the player, but is generally higher than prerendered subtitles. Also, some formats introduce text encoding troubles for the end-user, especially if very different languages are used simultaneously (for example, Latin and Asian scripts).

In other categorization, digital video subtitles are sometimes called internal, if they are embedded in a single video file container along with video and audio streams, and external if they are distributed as separate file (that is less convenient, but it is easier to edit/change such file).

Thanks for the info. I don't think it's 'normal' though, I've ripped hundreds of blu-rays, and most of them the subtitles are burned in fine with My Movies (Handbrake), it's just a few specific titles where it does not work and I have to resort to DVD fab and pick the subtitles to use.