These are the main/primary units you rez out for tenants to pay into for their rental.
CasperLet Rental Box (1 prim)
CasperLet Rental Meter (4 prim/2 impact)
CasperLet Rental Meter Mesh (1 prim / 1 land impact)
Slave/Relay Units
These are essentially "dumb terminals" - all they do is relay commands to the main unit they are linked to - you cannot use these by themselves, they must ALWAYS be linked to a respective main unit.