Red tag
Red tag turns up in 2 fields: electricians, metrology. Here is what it means to the people in each one.
Red tag in electricians
Failed inspection notice, or the act of an inspector failing work so it must be fixed and reinspected.
A red tag is the correction notice an inspector leaves when work fails, listing each violation to fix before reinspection, and as a verb it means failing the job. Some jurisdictions also red-tag unsafe equipment to forbid its use, and utilities will not energise a service until the tag is cleared. A green tag or signed card is the opposite, and nobody wants to explain a red tag to the boss.
"Inspector red tagged the basement for no bond on the gas line and two double taps, he's back Friday."
Red tag in metrology
A reject tag marking an instrument as unfit for use, or the act of pulling one from service with it.
A conspicuous red label hung on equipment that has failed calibration, is damaged or is overdue, meaning it must not be used until it is repaired, recalibrated or scrapped. As a verb, red-tagging an instrument pulls it from service and usually sends it to the quarantine cage. The tag is a visual control so nobody grabs a bad gauge from a bench drawer without seeing its status.
"Red tag that caliper, the jaws are worn and it read 0.05 over on the 25 mm block."