Apu Power Unit APU Shutdown Using GPU / ASU (Air Start Unit)?

APU shutdown using GPU / ASU (Air Start Unit)? - apu power unit

Several times the arrival of an aircraft is requested for beginners and down the message transmitted to the ground support-JET STARTER APU REQUIRED.

I understand the use of a boot drive in Jet flight, but why a startup called jet flights with Apu? Are used to cut the engines?

Sometimes even Ground Power Unit for the arrival of the aircraft known exactly why these things are down at the Aircraft APU, whose arrival means?

They are used to turn off the device's main engine?

Thank you in advance.