The System Reference Document--called the Black Flag Reference Document or BFRD--for Kobold Press' Black Flag roleplaying game system--launched as a reaction to the Open Gaming License crisis caused by Wizards of the Coast in 2022/2023--has been released under the Open RPG Creative (ORC) License, a viral share-alike license designed to replace the Open Gaming License.
A System Reference Document is a tool for third party developers, and contains the rules text that those creators are permitted to use, along with examples of how certain rules elements--such as subclasses--work.
The document includes lineages, heritages, classes, spells, monsters, and more.
The BFRD is the second major third-party 'alternate 5E' SRD to be released, following the Level Up: Advanced 5E SRD (A5ESRD) from EN Publishing.
Wizards of the Coast announced this week that the 2024 core rules of Dungeon & Dragons would be added to their own System Reference Document in 2025.