RJ9 connector is considered to be a 4 position, 4 conveyor plugs. It generally ends on a 4-conductor telephone link. RJ9 is basically used to connect a phone handset with a phone collector. The RJ 9 is not quite an ideal choice as a connector you will go over once in a while.
RJ9 is used for telephone handset cables, whereas RJ12 is used in the system phones. RJ9 has a 4P4C connector, whereas RJ12 has a 6P6C connector. RJ9 is the abbreviation for Registered Jack 9.
The difference between RJ9 and RJ10 is in the number of positions available on the connector and the number of contacts actually present. Obviously, where there are more positions, the connector is necessarily larger.