Many changes, including bug fixes and documentation improvements can be implemented and reviewed via the normal GitHub pull request workflow.
Some changes though are "substantial", and we ask that these be put through a bit of a design process and produce a consensus among the API Blueprint team.
The "RFC" (request for comments) process is intended to provide a consistent and controlled path for new features to enter the language.
You need to follow this process if you intend to make "substantial" changes to the API Blueprint language or specification.
In short, to get a major feature added to the API Blueprint language and/or parser, one must first get an RFC relevant to that feature merged and accepted into the RFC repository as a markdown file. At that point the RFC is 'active' and may be implemented with the goal of eventual inclusion into API Blueprint.
Please see RFC 1 for more details.
The RFC process is inspired by, and very similar to PEP, DEP and the Rust RFCs.