Parking Brake/emergency Spring Brake?

Even though I dont drive I deal with trucks on a daily basis. I use two knobs yellow diamond and red octagon. Parking brake and trailer what is a round blue one I see on the cdl practice test? Aren't the parking brake and emergency brakes the Same thing. I feel like I should know this but ive never seen this set up.

From Page 5.1 of the TT system

Dual parking control valves:

When main air pressure is lost, the spring brakes come on. Some vehicles, such as buses, have a separate air tank that can be used to release the spring brakes so you can move the vehicle in an emergency. One of the valves is a push-pull type and is used to put on the spring brakes for parking. The other valve is spring-loaded in the “out” position. When you push the control in, air from the separate air tank releases the spring brakes so you can move. When you release the button, the spring brakes come on again. There is only enough air in the separate tank to do this a few times. Therefore, plan carefully when moving. Otherwise, you may be stopped in a dangerous location when the separate air supply runs out.

This is the "blue button". You really don't see it that much anymore - especially in COMBINATION VEHICLES.

The only thing that releases the spring brakes on the trailer, is charging the TRAILER SUPPLY LINE (Red Stop Sign Valve).

Rick