When replication is set up, some databases on the publication server automatically have the repl_subscriber user added. This database user should not be modified, nor should users use it to access the database.
Replication processes from a subscription server are aliased to the repl_subscriber login ID. Replication processes from the subscriber server can execute stored procedures on the publisher in those databases that have the repl_subscriber database user. On a publication server, repl_subscriber is automatically added as a user in the master, distribution, and msdb databases and in all publication databases. (Note that two security elements are discussed here: the repl_subscriber login ID and the repl_subscriber database user.)
If the publication server is configured to use a remote distribution server, then the repl_subscriber database user is automatically added to the master, distribution, and msdb databases of the distributor.
For more information about replication, see Part 6, Replication.