Zonal RI’s

How do zonal RI’s work at the consolidated billing level, when AZ’s are not guaranteed to be the same across accounts?

Consider the following scenario:

Zonal RI’s are purchased for eu-west-1a (we will call this az1) from the consolidated billing account (account a).

Account b’s eu-west-1a abstracted az is actually a different backend az to the billing account and uses az2.

Are zonal RI’s placed logically? Or is it a physical reservation within an AWS facility?