CAT scales, at your local truck stop, typically have 3 separate zones (steer, drive and trailer).
To get tongue weight, it takes 2 passes; one with the trailer (distribution bars must be disconnected) and another with just the truck (no trailer). Tongue weight is the difference between "the truck" values (steer + drive) - (steer + drive).
I recommend skipping past any instructions that have you disconnect ON the scale unless you own the scale!