The Salsa RFC process is used to describe the motivations for major changes made to Salsa. RFCs are recorded here in the Salsa book as a historical record of the considerations that were raised at the time. Note that the contents of RFCs, once merged, is typically not updated to match further changes. Instead, the rest of the book is updated to include the RFC text and then kept up to date as more PRs land and so forth.
If you'd like to propose a major new Salsa feature, simply clone the repository and create a new chapter under the list of RFCs based on the RFC template. Then open a PR with a subject line that starts with "RFC:".
The RFC can be in its own PR, or it can also includ work on the implementation together, whatever works best for you.
Not all PRs require RFCs. RFCs are only needed for larger features or major changes to how Salsa works. And they don't have to be super complicated, but they should capture the most important reasons you would like to make the change. When in doubt, it's ok to just open a PR, and we can always request an RFC if we want one.