T480 has 2 RAM slots and can support 64Gb (32+32). The reason it's "unofficial" is that when they designed it (and wrote PSREF) the 32Gb RAM wasn't a thing.

My T480 running NetBSD: [ 1.000000] total memory = 65411 MB

T480 has 2 RAM slots and S has soldered RAM and 1 slot.

OTOH unofficially T480 (not S) can work with two 32 GB SODIMMs. I did not try that.

(I wish the AMD-based thinkpads supported ECC RAM. Ryzen 7 mobile CPUs technically allow for that.)