The repl_subscriber login ID is created automatically when a server is set as a publication server. It allows replication processes on a subscription server to connect to a publication server and execute stored procedures. This login ID should not be modified, nor should users use it to log in.
When replication is set up, a publication server is automatically configured as remote server to each of its subscription servers. Unless a previous mapping for the SA exists, when replication processes from the subscription server connect to the publisher, the SA account from the subscriber is mapped to the repl_subscriber login ID on the publisher. Also, repl_subscriber is set as a user in the master, distribution, and msdb databases and in all publication databases. This allows the subscriber to execute a specific set of remote stored procedures on the publisher.
If the publication server is configured to use a remote distribution server, then the distribution server is set as a remote server to the subscriber, the repl_subscriber login ID is created on the distribution server, and repl_subscriber is set as a user in the master, distribution, and msdb databases of the distributor. This allows the subscriber to execute specific remote stored procedures on the distributor.
For more information about replication, see Part 6, Replication.