Okay, I can explain this, having all the FCC licenses, both amateur and commercial. (There is a surprising amount of overlap between radio hams and pilots, same skill orientation, I guess.)
There are several different commercial licenses, a couple of which are designed for use on ships. Those aside, the one pilots have for international flights is the Restricted Radiotelephone Operator's Permit. As most respondents know, that just involves filling out a card and paying a fee, but it makes you legal.
The big step up from that is obtaining a General Radiotelephone Operator License. Basically this lets you repair aircraft transmitters. You need to pass 2 tests, which are given by FCC-approved exam groups called COLEMS. Each one costs about $30, plus the FCC gets another $60 fee. The exams are multiple choice tests and all the questions are on the FCC website. (Several books have both the GROL questions and answers.) So it parallels the FAA exam system. The first test is really easy and deals with basic regulations (don't swear on the air, know ship calling frequencies, etc.). That's required for a marine operator's permit, but if you get the GROL its privileges are folded in to it. The other exam is technical matters, so you need to know--or at least spit back--some across the board electronics. (And you can take another exam for a ship radar endorsement if you are interested in that technology.)
The licenses are all lifetime. Don't know if they are of any use except to show employers you've made an effort to learn more about systems, which I guess is a positive. (I'm a low time private so can't answer on what the lines are looking for.) Getting them are fun challenges if you're looking for something else to learn.