Custom sessions are available at the moment only for Seat Session-Based Application Version and allow you to link a single User with multiple IP Addresses to a running Deployment using a Custom ID provided by you.
The big advantage of Custom Session is that you do not need to sync Session ID and your User together. You can simply use your User ID as a custom ID and always use it to retrieve its current Session.
This also allows you to bind multiple IP addresses to a single entity without counting this User as multiple active sessions towards your Monthly Active User (MAU). Also, this allows you to update the IP Address List of the single User on the fly for Dynamic ACL.
At the moment there is no way to Deploy on-demand with Custom Sessions, and neither to do Live Telemetry to pick the best location. This is because a Custom ID is an abstract concept with no Physical Context. They need to be manually assigned to your running Deployment.
For a concrete example on how to use Custom Sessions, please refer to the API for more details.
Since Custom Sessions are always handled as a single User instead of a Match or a Party, you can POST and DELETE in bulk to simplify the process. Refer to the API for more details.