There are two storage scopes for persistent disks: Zonal and Regional. The figure below illustrates the storage scopes:

Figure 5: Storage scope types.

R1Z1/R1Z2 and R2Z1/R2Z2 are synchronous replications inside the same region used for HA. These disks are called ‘regional’ disks. Zonal disks do not have a secondary HA zone. R1Z1/R2Z1 is asynchronous replication used for DR applicable for both regional and zonal disks.

ARO does not manage failures related to synchronous replication (as of now). After a zonal failure, if it is desired to keep the VM in the same region, a procedure similar to the one described in the article Build HA services using synchronously replicated disks should be conducted, and then protection orchestration can be resumed by mapping the newly created VM onto the failed one. Asynchronous replication is expected to continue.

For example, if disk R1Z1 fails due to partial or complete zonal failure, a replica VM could be created using the R1Z2 disk, and then it could be mapped through the AROVA UI.
(See: Regional VM, primary region, primary disk’s primary zone(R1Z1) failure)