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.

• "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.

• "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.

Comparing USOC Jacks
USOC		Type of Jack		Electrical Network Connection		Mechanical Arrangement		Typical Usage
RJ-11C		Surface- or flush- mounted jack		Single-line bridged tip and ring only		Miniature 6-position jack		Single-line, non-key telephones and ancillary equipment
RJ-11W		Wallmounted jack		Single-line bridged tip and ring only		Miniature 6-position jack		Single-line, non-key telephones and ancillary equipment
RJ-48X		Surface-, flush-, or wallmounted jack		T&R, T1, R1, Conductors 7 and 8 provide cable shield integrity; Conductors 3 and 6 are reserved for future use		Miniature 8-position jack		1.544-Mbps digital services
RJ-45M		Surface-, flush-, or wallmounted jack		Multi-line bridged tip and ring		Up to 8 miniature, 8-position keyed jacks in multiple mounting arrangements		Multiple installations of programmed types of data equipment
RJ-45S		Surface-, flush-, or wallmounted jack		Single-line bridged tip and ring		Single miniature 8-position keyed jack for surface mounting		Programmed data equipment
RJ-48C		Surface-, flush-, or wallmounted jack		T&R, T1, R1, Conductors 7 and 8 provide cable shield integrity; Conductors 3 and 6 are reserved for future use		Miniature 8-position jack		1.544-Mbps digital services
RJ-48S		Surface-, flush-, or wallmounted jack		One- or two-line T&R or T&R, T1, R1		Miniature 8-position jack		Local area data channels/ subtrate digital services