EFIT is an equilibrium fitting code.
The most up-to-date code documenation is available publically at https://efit-ai.gitlab.io/efit/ which is built from files in the docs directory.
Legacy information about the development can still be seen at https://fusion.gat.com/theory/Efit (requires DIII-D cyber account) but is substantially out of date and incomplete. If you find something useful that is missing from the GitLab documentation please open an issue or pull request to have it added.