Field help (CHGTYPPTH) 

The exchange rate type proposed by default is the exchange rate type associated with the supplier at the origin of the receipt. An exchange rate type is used to determine a currency rate in the exchange rate table, taking into account the receipt date or the original order date, based on the PTHCURDAT - Date of exch. rate for receipt parameter value (ACH chapter, CUR group).

In a receipt, use this information to:

This information can be modified as long as no receipt line has been entered.

After creating a subcontract receipt, you can no longer modify the rate type.