The site version determines which Replication Server features you can use.
If, for example, you have Replication Servers with a site version set to version 15.0, you can create replication definitions for tables containing the bigint datatype. Such replication definitions are distributed to other Replication Servers with site versions that are at the same version level or higher. Replication Servers of earlier versions do not receive information about these replication definitions.
After you upgrade Replication Server and its user databases, and set the system version to the new version, you must set the Replication Server site version to the new software version level before you can use the new Replication Server features.
You cannot downgrade after you set the site version. You must set the site version for all Replication Servers that require it, and upgrade the routes. Information needed for the new features cannot be propagated to other sites until the corresponding routes are upgraded.