Distribution model mandatory

Hi Experts,

Can anybody please explain when the Distribution model is mandatory for IDoc generation?

I have done a set up in my sap sytem that as soon as PO is created, IDoc is generated.

But if I delete the distribution model, IDoc is not generated. But in some other cases, IDocs are generated without having a distribution model. Can someone pls throw some light on this.

Thanks very much.

Regards,

Rudresh