The primary member designation is stored in AD. The bit should only be set on one replication group member in the whole replication group for each replicated folder. When this member starts up fresh (i.e. no DFSR database) DFSR notes it has the primary flag set and will go through initial sync as the primary. This means the data won’t be fenced to lose and instead of waiting for to sync with other partners before allowing outbound replication the primary member will go online after the existing content has been walked and loaded into the DFSR database