The "Mechanics License" is an informal certification that all reputable technicians in the US employ and is widely recognized. It's a form of de-facto licensure that's easily recognized at a glancing distance, and serves to ensure that a mechanic is not going to leave on a moments notice, without at least bolting a wheel on. Instead, he or she will tell the foreman or whomever's name is on the sign, likely to call a tow truck for Friday.
It goes like this: Once a young apprentice lube tech graduates from Harbor F. University, an unusual ritual occurs in most mechanic's shops where a large, diesel, truck with a lift gate and a Square terminal approaches.
A nondescript individual emerges from the truck, (slowly, as to not alarm anybody who has a pending tab) where our young acolyte is soon taken gingerly by the hand and ushered onto this mysterious conveyance.
Seven or Eight hundred dollars later, the young, oily apprentice emerges holding a shiny, new, silver gun and possibly a ratchet and a t-shirt or pocket protector.
Most seasoned veterans will recognize this as their Mechanic's License. It's good for brake jobs, mostly.
If you foul up to many times in the shop, they call the MAC or Snap-On guy who will gun it in his diesel and demand full payment for the impact tool and ratchet. They usually get to keep the shirt though, because, yuck, you know?