What the tag records
A manufacturer's tag carries the part number, the construction description, the conductor size and count, the voltage rating, the length in feet, the gross and net weight, and a job or purchase order reference. On older reels it may be a stencilled flange instead of a paper tag, which survives weather better than most people expect.
Which fields carry the value
Length is the first field a buyer looks for, because a reel without footage is a construction with an unknown quantity. Voltage class and conductor material come next, then the construction description. The purchase order reference matters less for pricing and more for proving ownership and release when the cable came off a project.
When the tag is missing
Measure the barrel diameter and the build-up of cable on the reel, count the turns across the top layer, or weigh the reel and subtract an estimated reel weight. Any of those gives a defensible estimate, and stating it as an estimate is better than presenting a guess as a measurement.
Photographing the tag
Photograph the tag straight on, filling the frame, with the reel flange visible around it. A second photograph of the jacket print confirms that the tag still belongs to the cable on the reel, which is the one thing a buyer cannot assume.