Secondary is where the vehicles can stop to load/unload.
It can have four vehicles all park and load/unload at the same time but this is rare.
Since it is physically parallel to the main loading area it is similar in size but only the area for those 4 vehicles is accessible. Coming into it is room for another 4 vehicles or so to enter and wait, and though I've never seen more than two parked waiting to return, there's likely room for four to return to the ride as well.
As far as controlling it is concerned - vehicles can be tagged by the TMs in a number of ways. Departing either loading area they can be tagged to return to the primary belt, go to secondary, or be disabled (stay on main track but do not present itself at the load area - thus travel behind the wall.) Also from the control booth they can mark vehicles to enter the area. If someone gets sick on a vehicle and it's too much to clean up while it drifts through the main belt the TMs will tag it to disable it and then controls will send it to the secondary area (this keeps it from moving, just tagging it to secondary puts it in a show profile.)
Vehicles with regular guests are pulled out and sent to secondary so it can fill their seats. Also if a guest has trouble on the main belt but gets into the seat they'll have it unload at secondary.
Most days there's one or two vehicles regularly sent to secondary per cycle, but during slow times it can be none. Vehicles also go there so maintenance can check out harness and audio issues.
A vehicle entering secondary does a sort of backflip out of the last floo network scene - and because I've seen another vehicle fault in the station causing one to get stuck entering - it puts guests at a full 90 degrees sideways briefly. The in/out of secondary is a small aerobatic feat not seen in the rest of the ride.
Architecturally it's stone work with the mirrors as the main load platform but in a minimalist style that "fades out" on the ends into dark space. Those dark nothingness areas are where vehicles wait to enter or exit its main load area.
The vehicles presents itself, parks, and then drops a few inches so guests feet can touch the ground. Unload/load occurs and then the vehicle is tagged with a profile/destination and then the TM tags the corresponding marking in the wall to the vehicle parking spot to send it back into the ride.
They'll lift up, drift sideways, the floor drops off and they enter the void area, and then they wait for a bubble in the chain to open up. When a slot is available in the main chain they'll swing into the ride at the first floo network. They skip Herimione's whole "Observatory!" thing.