Hi Hussain,
On the split tab, what you see is the Account Assignment Group (AAG) which pulls in the AAG from the Payer, which you enter on the Customer field of the Split.
Like in SD, even here, you would see all 4 mandatory partner functions (Sold to, Ship to, Bill to & Payer).
If you test with an example (if the account groups are already there, then create a single sold to without any other partner functions / use a different payer than what you have from sold-to on order and then check), you would be able to realize that, you cannot create a split to sold to (if this sold-to has no payer assigned on the BP/Customer Master). That would help you determine if the AAG is getting populated from Sold to / payer.
I hope this helps.