Z registers are pseudo-registers used only briefly during code generation.
Their main purpose is to prevent allocation an X register for an SSA instruction when the corresponding BEAM instruction does not return a value. Allocating an X register that will not actually be used by the BEAM instruction could result in either unsafe code or code with more register shuffling because an X register seems to be occupied while it’s actually free.
I might also have used Z registers to transfer values between two consecutive SSA instructions that are combined into a single BEAM instruction.