"Not a mariner yet, but from 46 CFR 12.403:
(1) Able seaman—unlimited. Three years of service on deck on vessels operating on oceans or on the Great Lakes.
(2) Able seaman—limited. Eighteen months of service on deck on vessels of 100 GRT or more which operate in a service not exclusively confined to the rivers and smaller inland lakes of the United States.
To me it just seems like less experience on vessels in a certain body of water."
You are correct that those are the requirements, I was just referring to what the limited vs. unlimited means pertaining to where and, on what tonnage you can work.