The Universal Service Ordering Code (USOC) system was developed by the Bell System and introduced by AT&T® in the 1970s to connect customer premises equipment to the public network. These codes, adopted in part by the FCC, Part 68, Subpart F, Section 68.502, are a series of Registered Jack (RJ) wiring configurations for telephone jacks that remain in use today.

Registered Jack numbers end with a letter indicating the wiring or mounting method being used.

• "C" —Identifies a surface or flushmounted jack.
• "W" —Identifies a wallmounted jack.
• "S" —Identifies a single-line jack.
• "M" —Identifies a multi-line jack.
• "X" —Identifies a complex multi-line or series-type jack.

You'll also often see these terms associated with USOC:

• "T/R" —Denotes connections to the tip and ring wires of a telephone communications line, trunk, channel, or facility.
• "A/A1" —Signifies connections to the hold functions of key telephone systems. The "A" lead corresponding to a particular telephone line is shorted to the "A1" lead when that line is off-hook in order for that line's "hold" functions to operate correctly.
• "Bridged" —Signifies a parallel connection.
• "Data" —Data configurations use jacks that incorporate components to limit signal power levels of data equipment.

If you (the telephone subscriber) want to have a standard jack installed other than the RJ-11W or RJ-11C, you should specify the appropriate USOC when requesting the installation.