They don't tell you what the qualifications are.
Just a guess, but they're probably not entirely different than the requirements for a TWIC Card, also issued by the TSA to gain access to secure areas such as maritime ports, refineries, and other "sensitive" locations.
I do have a TWIC card. It's always interesting (scary?) when I present the federal credential to a TSA agent at an airport. Although the TWIC card provides no access to airport facilities, it is a valid form of government ID issued by the TSA, Most agents are familiar with it, but one agent, after checking my boarding pass and waving me on asked me "what is this TWIC thing, anyway?"
Security theater!