What is the System Reference Document (SRD)?
It's basically a Open Gaming License subset of what's in the PHB, DMG, MM, Epic Level Handbook and the Psionics Handbook, the real difference being that what Wizards of the Coast has declared as Product Identity (PI) or Closed Content. That means the deities are not in there, some monsters, character names and places are omitted, and spell names may have been changed.
See WotC's declaration what is not Open Content here: SRD Legal Information
All non-SRD material (e.g. the splat books: Complete Warrior, Defenders of the Faith, Sword and Fist, Forgotten Realms, Dark Sun, etc. all TM of Wizards of the Coast) was handled by Code Monkey Publishing (see the next item).
Why is book XYZ not included in PCGen?
There are a few possible causes for this situation and the explanation hold true for both datasets that have never been in PCGen and some which have been in PCGen but were removed.
For more information on what sources are available in PCGen and explanations for those that are not, please refer to our Publisher Permissions page with this documentation set.
Where are Wizards of the Coast's books and modules?
Those had been handled under a special license by the gang at Code Monkey Publishing (CMP). Sadly, late in 2006, WOTC declined to renew CMP's licence, and so they are no longer available for distribution.
Where are the Star Wars/Call of Cthulhu/Wheel of Time files?
Even though Wizards of the Coast produces those books, they don't own exclusive rights to that material. We'll need to get permission from both Wizards and the other copyright owner(s) of said material before we can include them in PCGen. It is our intention to acquire this permission, but we're being very careful about how we proceed to maximize our chance of success. We'll keep everyone informed when anything worthy of reporting occurs. :)
How do I include sources from Game Mode X in Game Mode Y that I currently use?
It is possible to do this, but you need to understand that this may cause errors due to incompatibility of the different Game Modes..
To include all of Game Mode X's sources in Game Mode Y.
Now, all of the sources associated with Game Mode X will show up in Y. Remember you may experience problems while doing this.
To include specific sources from game mode X in game mode Y.
Now, all of these specific sources from game mode X will show up in Y. Remember you may experience problems due to Game Mode incompatibilities while doing this.