I think you might be incorrect w.r.t. it being a 'rating' under the FAA system.
AFAIK it's a class on a certificate no different from the Single Engine Land or Multi Engine Land. What's more it's independent of your other certificate(s).
You could, for example, hold a PPL-SEL, a CPL-SES and an ATP-MEL. None of them have crossover privileges to the other certificates (except perhaps the IR privileges inherent in the ATP and the usual 'lower level of licence privileges are embraced by higher level licences' thing). In this example if you fly a SEL class of aircraft you may only exercise PPL privileges. If you wish to exercise commercial privileges then it must be in either an SES or an MEL class (the ATP embraces CPL privileges). If you wish to exercise ATP privileges then it must be in a MEL aircraft.
The flight test ('checkride' in US parlance) is a test for the issue of an FAA certificate even though the syllabus recognises skill transfer from other certificates (the 'add-on' certificate terminology that you might hear used). This means that you will have to satisfy the TSA's insecurity stupidity.