If you go back to the old Amiga days, they used to have
chip memory and fast memory. The FAST RAM could only be accessed by the CPU, whereas the Chip memory was accessed by the rest of the computer, notably by the graphics system.
Just a thought but if they are going to allow RAM upgrades this RAM could be accessible by the CPU only and not be a part of the unified RAM setup that the graphics could access.
For example, this could be something that a Apple could do with the 27" iMac. Make all SKUs 16Gb - theoretically maxing out the on-package unified RAM on the M1. Anything added via the slots would then be accessible by the CPU only.
I expect this could be the model followed by high end Mac mini Pro and iMac for example. Max out the M1 RAM and anything after that is for the CPU only.