Sign In API
Where the Sign In operations of the Platform API and the legacy Sign In API are documented.
The Sign In operations of the Platform API are under Platform › API:
Base URL, IDs and authentication are in Introduction and Authentication.
A Sign-In Context that uses the legacy Sign In API with its Legacy API key keeps it until the API is turned off. It cannot be turned on for another context, or again after it is turned off. Its operations are in the legacy Sign In API, and Migrate from the legacy API maps them to the Platform API.