The substrate selector consists of a nucleoprotein complex (also called relaxosome) formed by an oriT, a relaxase and one or more accessory nicking proteins.
The relaxase protein specifically cleaves oriT in the DNA strand to be transferred, remains covalently bound to the cleaved strand, and presumably religates it at the end of the transfer process.
Conjugative relaxases are related to rolling-circle replication initiator proteins, and their target oriT sequences are related to rolling-circle replication oriV sequences.
The selector is so called because it is very specific for each plasmid system. Relaxases act specifically on their cognate oriT, and not on those of related plasmids.