Before tatts became mainstream, you could tell what prisons that convicts had been at and what they were doing time for by their tatts.
For a while during the Vietnam War, tatts were frowned on by the military because some tatts would enrage the VC and cause them to kill you on sight. Tatts that identified units or service branches were banned. Many units had no tatt policies.
The Navy and Marines have the longest traditions with tatts. Many naval and marine units had special tatts like the Marine jungle Tiger that only Guadalcanal vets could wear.