It's not written in to the spec for fairly straightforward reasons - UEFI as a body is really only empowered to specify mechanisms, not how they're used. That's the power the hardware (and software) vendors give them.
MS gets the power to specify how the mechanism is used, because MS is in a market position to make that kind of demands and back them up with their marketing dollars. UEFI has no economic carrot, nor a stick, which is why UEFI wasn't (very) widely deployed on systems until MS said vendors had to to get Windows 8 marketing money.
Re: Why's it not in the spec
MS gets the power to specify how the mechanism is used, because MS is in a market position to make that kind of demands and back them up with their marketing dollars. UEFI has no economic carrot, nor a stick, which is why UEFI wasn't (very) widely deployed on systems until MS said vendors had to to get Windows 8 marketing money.