Reference Designators label the location on a wiring harness to ensure assembly is straightforward on the aircraft. The reference designator usually consists of one or two letters followed by a number. The convention of Plug (P) and Jack (J) when assigning references for electrical connectors in assemblies where a J (or jack) is the more fixed and P (or plug) is the less fixed of a connector pair, without regard to the gender of the connector contacts.
Shrink tubing is used for harness identifiers, reference designators, cover for wires, and in solder sleeves and splices. The common material for shrink tubing is polyolefin. Shrink tubing is manufactured at a small diameter then is expanded to a larger diameter. When heated, the tubing ‘recovers’ to the predetermined diameter. Most shrink tubing only change in diameter, not in length. Most aircraft shrink tubing will recover to ½ of the diameter it started. Some of the more expensive tubing will recover to ¼ of the diameter it started. Typical shrink tubing part numbers begin with M23053, P5034, P5381, and P5382.