USOC, Universal Service Order Codes, were developed in the 1970's by AT&T to identify tariffed services and equipment. These codes were lated adopted in part by the FCC, Part 68, Subpart F, Section 68.502.

USOC codes, as applied to telephone jacks, are explained thusly;

• RJ - denotes registered jack
• C - flush or surface mount jack
• W - wall mount jack
• X - complex multi-line or series type jack

Pin Designations

• Pin location for 6 and 8 conductor modular jacks and plugs
• Standard wiring patterns for 6 pin modular jacks.
• Standard wiring patterns for 8 pin modular jacks.

Each jack has a specific application.

• Single line phones, accessories, answering machines, and modems use the RJ11C or RJ11W jack.
• Two line phones, accessories and answering machines use the RJ14C or RJ14W jack.
• Three line phones and accessories use the RJ25C jack.
• Four line phones and accessories use the RJ61X jack.
• Burglar and fire alarms circuits use the RJ31X or RJ38X jack.
• Single line fixed loss loop data installations use the RJ41S or RJ45S jack.
• Four wire data circuits use the RJ48C/RJ48X or RJ48S jack.
• One to twenty five single or multiple line circuits bridged to the network or customer equipment use the RJ21X.
• RJ22 is the designation for the smaller jack that is used on handsets.

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