How system pick partner profile in order/invoice

Hello Experts,

In invoice [VF03], I could see system determine output type RD00 and partner function BP so system is taking Bill to party from invoice and putting there. I checked the output condition record for that invoice [VV33] there there partner number is not assign to partner function BP. I understand this.

BUT my questing is, How system is picking partner profile to create the Idoc for that invoice??

Partner profile and Bill to party both are different.

How Idoc creating process start when we save sales order/ Invoice ???

I want to know the triggering point from where system start idoc creating for sales order/ invoice ???

Thanks