Used with global transaction ID to identify logically independent replication streams. When events can propagate through multiple parallel paths (for example multiple masters), each independent source server must use a distinct domain_id. For simple tree-shaped replication topologies, it can be left at its default, 0.
Additional information on this topic may be found in the MariaDB Public Knowledge Base.
Information specific to MariaDB SkySQL can be found on the gtid_domain_id page in the SkySQL Documentation.