None?
They aren't paired in any way, except you are only able to order what your computer originally came when it comes to storage through the Self Service Repair shop.
Apparently the NAND modules used in the M2 Max and M2 Ultra Mac Studio and Mac Pro (2023) are the same, so you can just buy them directly from Apple or 3rd party retailers (2TB, 4TB and 8TB available).
OK, I took another look, and I can see that parts pairing as a term of art is very specific--it's about allowing a part that will work in one Apple device to work in all devices of the same model, regardless of the serial no.
I was thinking of pairing in a different sense--namely the restriction that prevents 3rd parties from making after-market slotted SSD modules: Currently, the only modules that will work are those that have the circuitry that enables the part to pair with the machine.
This is not part pairing based on machine serial no., it's part pairing based on being an Apple OEM part. That's why the only way the modders can make this work is to solder larger NAND chips into Apple OEM modules.
Given that the point of this legislation was to allow 3rd party parts to work, it seems narrowly restricting it to those defeated by serial no. specificity makes it too narrow--you're still restricted to using Apple OEM parts only (or, in the case of the modders, modifying Apple OEM parts).