Why Backorder or rescheduling not possible in 01(CG)

Hello Gurus,

i m new to this forum as well as new to sap sd ....when i was practicing always in mind 3 things in this course, what.. when.. why..

Today i saw backorder scheduling & rescheduling is not possible for 01 checking Group(MRP3) ...so just need to know why its not possible ? for this is there any fields or controls which maintaining this ???if yes where the control for this ?

And when backorder and rescheduling is possible and why ? Please explain ..full explain

Thanking You

Manas Kumar😊