These 2 threads may give you more information. There is an algorithm for load balancing but i'm not sure if it's documented.
https://social.technet.microsoft.com/Forums/en-US/e7af1df8-0871-49a3-b865-41f574e05c5e/rds-2016-load-balancing?forum=winserverTS
https://social.technet.microsoft.com/Forums/windows/en-US/72b9ee91-5822-42a0-be53-68df5fd3a6d9/some-questions-about-a-new-rds-farm?forum=winserverTS
hth