The standard keeps global settlement unambiguous. USD is US plus dollar; JPY is Japan plus yen; CHF uses Confoederatio Helvetica, the Latin name for Switzerland, plus franc. Supranational currencies break the country rule: EUR for the euro, XAU for gold measured in troy ounces, XAG for silver.
Trading platforms use these codes everywhere, including for metals quoted like currencies. XAU/USD is an ounce of gold priced in dollars and behaves like an FX pair on most retail systems.
Example: a platform lists XAG/USD at 31.20. That is one troy ounce of silver for $31.20, and a 100-ounce position moves $100 for every $1 change in the price.
Related: currency-nickname, currency-pair-notation