Nomenclature
Modular connectors also go by the names "modular phone jack/plug", "RJ connector," and "Western jack/plug." The term "modular connector" arose from its original use in a novel system of cabling designed to make telephone equipment more modular. This includes the 4P4C handset connector which is sometimes called an "RJ9" or an "RJ22" — but such RJ specifications are absent in the current FCC documents.
It is very common to use a registered jack number to refer to the physical connector itself; for instance, the 8P8C modular connector type is often called RJ45 (after a telephone company standard that uses a similar connector), but 8P8C connectors are also used with different signals, such as for Ethernet over twisted pair.
Read more about this topic: Modular Connector