Being signed off as a Trainer implies that you can teach all spotter courses and train/spot both Class A and Class B Flyers.
To become a Trainer, you have to shadow and assist an Examiner on each of the 3 spotter courses. Only an Examiner or Chief Examiner from the organization can then sign you off as a Trainer if you performed successfully. A Trainer is then allowed to train tunnel instructors of all levels, but is not allowed to sign them off as such. This will again be done by an Examiner or Chief Examiner when they successfully pass a final test.