To stop the master caution! ;)
But seriously: When it comes to creating a realistic startup-procedure, you'd let the APU + APU GEN running until R ENG is up, unless you want to perform a X-Feed Startup? Then shutting down the APU GEN and about 1-3 minutes after the APU? I read somewhere, that IRL you have a delay between shutting down the APU GEN and the APU, due to cooling of the APU. Is that correct?