When Lightyear creates a map, the currency defaults to the national currency of the supplier. So if it is an Australian supplier, the map will be created in AUD. But what if that supplier does not bill you in AUD?
When you Find Map > Try Map > Use Map from the Processing tab, you will then need to either assign that map to an existing supplier (from the dropdown menu) or create a new supplier. As part of the process of creating a new supplier, you set the default GL codes, categories etc. As part of that process you will also be given the option to set the default currency that this supplier bills you in.
You can of course choose the currency on a bill by bill basis by changing it in Panel 2 of the Approvals Tab or Review Tab.
You can also change the default currency at any time in the Suppliers section.