This determines which currency symbol displays, NOT the number of decimal places that display, not the decimal character, or whether you use commas or periods for thousands separators (that’s covered by Locale, which sets the number format).
This is also the currency used for roll-up reports so transactions in other currencies are converted to this one. Currency financials display in (funds etc). That’s out of scope for the feature that adds the setting, but might be worth noting for context.
See attached zipped folder Currency Mockups.zip for Kimie Kester's latest mock-ups.