Copying the relevant docs from Upgrades:
By default, upgrades use the main prestige currency for the layer. You can include these to change them (but it needs to be a Decimal):
- currencyDisplayName: optional. The name to display for the currency for the upgrade.
- currencyInternalName: optional. The internal name for that currency.
- currencyLayer: optional. The internal name of the layer that currency is stored in. If it’s not in a layer (like Points), omit. If it’s not stored directly in a layer, instead use the next feature.
- currencyLocation: optional. If your currency is stored in something inside a layer (e.g. a buyable’s amount), you can access it this way. This is a function returning the object in “player” that contains the value (like