In terms of how the memory is allocated, games get a total of 13.5GB in total, which encompasses all 10GB of GPU optimal memory and 3.5GB of standard memory. This leaves 2.5GB of GDDR6 memory from the slower pool for the operating system and the front-end shell. From Microsoft's perspective, it is still a unified memory system, even if performance can vary. "In conversations with developers, it's typically easy for games to more than fill up their standard memory quota with CPU, audio data, stack data, and executable data, script data, and developers like such a trade-off when it gives them more potential bandwidth," says Goossen.
There is no other GPU Optimal Memory reservations based on the statement: "nearly the full 10 GB (10208 of 10240 MB) of GPU Optimal Bandwidth memory is now available to titles".
I have assumed the same or possibly less than 32 Meg applies to the Series S -- it all depends if they freed any up for the Dashboard assuming the Series S targets lower resolution than Series X.
There were some other interviews and technical presentations that for the Series S developers do not need to be concerned about the slower memory pool as that is only ever used by the OS. This would place OS Reservation at 2.5 GB for Series X and 2 GB for Series S from the slower memory segment combined with the 32 Meg Series X and 16-32 Meg Series S from GPU Optimal Memory.