Advance Invoice vs Prepayment Based Invoice

@runise At the moment, you can’t use both options for a given Company (This is same even in Latest Cloud version 22R2 it seems). See below extract from IFS Help Docs,

• The value you select in the Method for Invoicing Prepayment field will decide how the invoicing of prepayments will be handled. The available options are: Advance Invoice and Prepayment Based Invoice.
  • If Advance Invoice is selected, advance invoices will be created for requiring prepayments, i.e. before receiving the prepayment. 
    • The base value retrieved on the advance invoice will depend on the value selected in the Base For Advance Invoice list.
  • If Prepayment Based Invoice is selected (typically used in Poland), invoices will be created after the prepayment has arrived. Note that Prepayment Based Invoice can only be created for the company working with single tax code per line items.

I don’t think any (Or not many) of the existing IFS Customers see this (I.e. using both options at the same time) as a valid business requirement, and that may be why the same distinct options were there across all past IFS versions and remained unchanged even in Cloud. So, I suggest you log this as a Support Case/an ‘Idea’ so that your (or your Customer’s) requirement/request is considered when implementing similar functionality in next Core version of IFS Applications.