World Sales Question. P42997 Invoked by P4211-

I'm using World A73 cum 11 and sometimes I've found SO lines cancelled (status 980/999). After checking the data in F4211/ F42199 I found that they have been canceled by P42997 invoked by P4211.

But it is not always but only in some conditions I did not understood.

Please note that in my environment either parts either customer are set to allow the back order.

So my questions is: why/when P42997 is invoked by P4211 and cancel the SO lines ?

Thank you

Silvio