a writes:

As a matter of minor interest, there's an interesting logic trap in
play here. I think it is safe to say all holders of the airline
transport rating also hold commercial certificates at least in the US.

Since the commercial certificate is subsumed in the ATP, your assertion is
universally true.

The FAA issues several types of certificates, and each type main include
multiple ratings. The pilot certificate types are as follows:

Student
Sport
Recreational
Private
Commercial
Airline Transport Pilot

(There are many other non-pilot certificates, such as mechanic and
dispatcher.)

For each certificate type, you can hold several ratings. Examples of ratings
include:

ASEL - Land airplane single-engine
AMEL - Land airplane multi-engine
INSTA - Instrument airplane
GL - Glider
HEL - Rotorcraft helicopter

You can be a student pilot for gliders, for example, while being a commercial
pilot for multi-engine airplanes.

Some pilot certificates implicitly include the privileges of other
certificates. For example, an ATP includes commercial and private privileges,
and also includes an instrument rating (the only pilot certificate for which a
separate instrument rating is not necessary).

I don't know why the FAA has made it so complicated, but it has.