While the source code is the best bet here as @tonyclewis pointed out , we do have some documentation from very early on in the development of the RestAPI module in the docs/dev folder inside of the repo, which can be viewed at https://github.com/FreePBX/restapi/tree/release/13.0/docs/dev. Note that these docs may be out of date and need updating, but should be a decent starting point. If you find anything that is out of date, by all means feel free to open a pull request.