In AeroATC, a top-down system is used, in which a higher-ranked controller can assume the positions below them. For example, if a Center C1 controller is online, they may also assume the functions of S1, S2, and S3 for airports that do not have these positions online. 

C1 Center Controller: responsible for controlling an area control center and routing aircraft during the mid-phases of their flight, after they’ve been handed off by departure, or forwarding a pilot to approach.
S3 Approach, Departure Controller: controls aircraft arriving and departing an airport.

S2 Tower Controller: clears aircraft onto the runway for crossing, take-offs, and landings. Additionally, responsible for aircraft performing traffic patterns.

S1+ Visual Tower Controller: visual control tower at single-runway airports.

S1 Delivery, Ground: gives pilots flight clearance and controls aircraft on the ground (except for the runway). 

TR Trainee: essentially someone with no rating, undergoing training or waiting for an S1 evaluation.

As a trainee, you will begin as a TR and train to become an S1 controller and move your way up, going through training and evaluations for each rank.