in the xena release we adapted placement to support consumer types so we could differentiate between allocation that were held by vms and allocations that were headed by migrations.

https://docs.openstack.org/placement/latest/specs/xena/approved/support-consumer-types.html

in the queens release we introduced migration allocations

https://specs.openstack.org/openstack/nova-specs/specs/queens/implemented/migration-allocations.html

currently evacuate does not use migration allocations whereas all other move ops that require multiple claims do. currently, no move operation uses consumer types.

as a result, the operational impact of failures is increased as its harder to reconcile
mismatches. the use of a single allocation for evacuate also breaks it in some cases such as vgpu https://bugzilla.redhat.com/show_bug.cgi?id=2128568

or where