This article addresses the naming requirements for Sub-TNP cluster and Sub-cluster when they are created from NSX Manager directly in a SDDC managed environment.
VMware Cloud Foundation 5.x
Broadcom recommends to keep the names identical for both Sub-TNP cluster and Sub-cluster, if they are being created from NSX directly.
Sub-TNP cluster and Sub-cluster are referenced in the Cluster Expansion workflow as 'networkProfileName'. Hence, cluster expansion workflow recognizes Sub-cluster through name matching with Sub-TNP cluster. This is by design.
Reference API specification: https://developer.broadcom.com/xapis/vmware-cloud-foundation-api/latest/data-structures/ClusterExpansionSpec/