For foreign vendors, this option is available from the Actions button when viewing or maintaining AP bills, or viewing vendor activity. If the bill is for a foreign vendor, you can click on this option to display all amounts in domestic currency.
If you select this option and the vendor's currency is different than the system currency, "Domestic" appears in the upper right of the screen and the program switches to read-only mode. When you click on Toggle Currency again, the display returns to the foreign currency and you can edit the fields again (that is, assuming you accessed in entry mode).