Maximum number of parallel threads to use on slave for events in a single replication domain. When using multiple domains, this can be used to limit a single domain from grabbing all threads and thus stalling other domains. The default of 0 means to allow a domain to grab as many threads as it wants, up to the value of slave_parallel_threads.
Additional information on this topic may be found in the MariaDB Public Knowledge Base.
Information specific to MariaDB SkySQL can be found on the slave_domain_parallel_threads page in the SkySQL Documentation.