How does cardiff uni accomodation application work?

Might be a silly question, but how does the application for student accomodation work? For my firm, I got to rank accomodation buildings on what I wanted, but for cardiff I don't have a choice of any of the buildings, only types (self catered, contract length and catering). It also says i need 2-5 but I can't choose the same option (ensuite, shared bathroom) twice. How exactly does this work?