Glossary

The following is a very simplified glossary of DVD terms designed to help you with the more important concepts. A more complete one is set out here.

Introductory concepts
VTS	Video Title Set	A group of related video titles having similar display format (standard screen or wide screen). VTSs contain PGCs which contain all the DVD’s titles and menus. A VTS contains a VTST domain, which contains the VTS’s titles and (optionally but very usually) a VTSM (menu) domain.
PGC	Program Chain	Each VTS contains one or more PGCs. A PGC is a collection of Cells, grouped into Programs (and chapters in the titles domain), or can be a “dummy”, which contains no video content, only command instructions. A special PGC is the First-Play PGC, which is the PGC a player looks for when the DVD is loaded. It is contained within VIDEO_TS.IFO and only contains command and branching instructions to start off the DVD's navigation.
Registers
GPRM	General Parameter	16 memory locations in the DVD player, which can hold numbers from 0-65535. Used to test values to determine branching conditions in the DVD’s commands.
SPRM	System Parameter	23 special locations in the DVD player, which hold specific information such as preferred languages and display mode, region code highlighted button and playback information. More information here.
Structure of VOB files (physical organisation of content)
VOBU	Video Object Unit	The smallest accessible building block on a DVD. It starts with a Navpack and includes about 0.5 seconds of video (and related audio and subpicture) content.
Cell		Cells contain any number of VOBUs and are given a unique VOB ID and Cell ID number. All the cells in a particular domain together make up the VOB file(s) for that domain. VOB files must be less than 1Gb for backwards compatibility reasons.
VOB	Video Object	Distinct from the file type of the same name, a VOB is a collection of one or more cells. Each VOB has a unique VOB ID number. Together with the Cell IDs, the various VOB IDs make up the video content.
Logical organisation of a DVD
Domain		There are 4 domains on a DVD: First Play (contained in VIDEO_TS.IFO and can not contain any video) Video Manager (VMG) VTS Menu (VTSM) VTS Titles (in the VTST area)
VMG	Video Manager	Consists of control information for the DVD. This is contained in the commands and structures in VIDEO_TS.IFO and, if video content is present, VIDEO_TS.VOB VMG menus contain the Title (or Top) Menu, if any, as well as other PGCs, which are used to link to various parts of the DVD. See the Jumps and Calls Reference here.
VTSM	Video TitleSet Menu	The domain in a VTS which contains the menu PGCs. The menu domain may contain any or all of the following defined menus: Root menu Chapter menu Angle menu Audio menu Sub-picture menu as well as other PGCs which either contain or do not contain video content. The actual video content for the menu will be contained in the VTS_xx_0.VOB file.
VTST	Video TitleSet Titles	The domain of a VTS which contains the title PGCs. The video content for the titles comprised of all the cells for that domain are contained in VTS_x_y.VOB (where y is 1-9 and x is between 1 and 99).
PGN	Program Number	A Program is a collection of cells. In any PGC, programs are numbered sequentially from 1. In a DVD movie, a Program is typically (but not always) one cell.
PTT	Part of Title (Chapter)	A PTT is a collection of Programs. PTTs start from 1 and are sequential in one-sequential titles. In not one-sequential titles, the PTTs can be spread over various PGCs.
Other
PUO	Prohibited User Operations	User operations which are prohibited from being executed (e.g. fast forward, next chapter). These PUOs can exist on a VOBU basis (in the VOB files), or in the PGCs (the IFOs) and some even in the title play map table (part of VIDEO_TS.IFO). More information here.
LU	Language Unit	An LU is a way of separating menus in the same menu domain for different languages. If there are several LUs, each containing menu data, the player will select the LU based on the value in SPRM (0) – Preferred Menu Language. If that language is not present, the first LU will play.
Commands		Any PGC can contain DVD commands. Commands are executed at 3 times in a PGC: Before video is played (pre-commands) After a cell has played (cell commands) After all the cells in a PGC have finished playing and the cell commands, if any, have been executed (post commands) Sometimes, the post commands will not be executed due to branching via cell commands. There is a special command set for DVDs, a summary of which can be found here.