This use covers both the way of testing and the test itself, so you can say the exam is now a CBT and also that the school has moved to CBT. It usually means you go to a test centre, sit at a screen, and answer there, which is why it comes up in booking instructions and exam rules. It sits very close to the training sense, and the difference is simply what the computer is for: a CBT measures what you already know, while computer-based training teaches you something new.

You meet this mostly at work, where staff finish a set of screens and questions before they are allowed to start a job or use a machine. It is uncountable in this use — you do CBT, complete your CBT, buy CBT software — and it often appears in front of another noun, as in CBT module or CBT package. Because the letters stand for several different phrases, the room you are in is what tells you which one is meant: in an office it is usually training, in a clinic it is therapy.