Hi-vis classes are written into ANSI/ISEA 107. The class number relates to the amount of background fluorescent material and reflective tape required, which scales with the speed of traffic around the worker.
Class 1. Lowest. Mainly for parking-lot attendants, warehouse workers in low-speed indoor environments. Honestly we rarely sell class 1 — most jobs that need any hi-vis need at least class 2.
Class 2. Required for workers in environments with traffic moving 25-50 mph. Surveyors, school crossing guards, airport ground crew, utility on side streets. A class 2 vest by itself is enough.
Class 3. Required for workers in environments with traffic over 50 mph or in conditions that reduce visibility (night, fog). Highway workers, emergency response, flaggers. Class 3 means the upper body AND the arms or legs are covered with the right material — so you'll see class 3 jackets with sleeves, or class 2 vests paired with class E pants to make a class 3 outfit.
Class E. The pants designation. Class 2 vest + class E pants = class 3 ensemble. This is how a lot of road crews comply.
What foremen actually buy. Class 2 vests are the high-volume sellers. Class 3 long-sleeve shirts move heavy in winter when guys can't keep a vest on over the layers. Class 3 jackets for night work. We stock all of it — also through fleet accounts (/services/fleet-and-crew-accounts) at volume pricing for crews of 5+.