As discussed in this blog post, PSC replication primarily involves the vmdird - VMware Directory Service.
This VMware Directory Service provides a multitenant, peer-replicating LDAP directory service that stores authentication, certificate, lookup, and license information. If your domain contains more than one PSC or embedded VC instance, an update of vmdir content in one vmdir instance is propagated to all other instances of vmdir.
All of this vmdir information is stored in a data.mdb file. This data.mdb file and its contents are essentially what are replicated.
The size of this file is usually about 15-20MB per node – it really should not be beyond 150-200 MB in 99% of the cases.
How Replication is Broken
There are two ways that we see vmdird replication breaking: