Custom sessions allow you to link a single User with multiple IP Addresses to a running Deployment using a Custom ID you provide. Currently, this functionality is only available through a [Seat Session-Based Application Version] (/docs/deployment/session/more-in-depth#the-different-session-kinds).
The significant advantage of a custom session is that you do not need to sync Session ID and your User together. You can use your User ID as a custom ID and always use it to retrieve its current Session.
Moreover, this customized ID allows you to bind multiple IP addresses to a single entity without counting a given User as multiple active sessions towards your Monthly Active User (MAU) count. Plus, 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 launch deployments on-demand with Custom Sessions, nor use Live Telemetry to pick the best location. These limitations are because a Custom ID is an abstract concept with no Physical Context. They need to be assigned manually to your running Deployment.
For a concrete example of using Custom Sessions, please refer to the API for more details.
Given that we treat custom sessions 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.