For others, the SRD is the System Reference Document. It is the core of the rules, with a few classes, a few spells, and so forth. The basic framework of the D&D rules. Since 3rd Edition, Wizards of the Coast (the D&D company) has offered it free of charge on a special free-use license (Open Gaming License.) It serves as a reference to the rules, as a sort of free demo, and (most importantly) it allows fans and other companies to write content using their rules without running afoul of their lawyers. As an example, anyone can use the OGL/SRD to write D&D compatible adventures or supplements as long as they don't mention the name of anything copyrighted that isn't in the SRD. Anything not included in the SRD, including many races, classes, abilities, spells, monsters, items, and so forth are off-limits. This is why you see tons of products advertised as being "Compatible with the world's oldest role playing system" (the term "Dungeons and Dragons" being copyrighted and not in the SRD.) Nothing, however, stops an author from creating new classes, races, rules, etc to flesh out the SRD stuff. A prominent example would be Pathfinder, which used the D&D 3.5 SRD as the core for their system, then built the rest on top of it.
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