A unit of account is the measure in which prices, debts and financial records are expressed. Using one common unit makes unlike goods and obligations comparable without quoting an exchange ratio for every possible pair.
Pricing and payment can differ
A shop can price an item at 100 US dollars while accepting bitcoin at the exchange rate quoted for checkout. The dollar remains the unit of account for that price; bitcoin is the payment asset. If the bitcoin price is fixed instead, the seller's dollar proceeds can change with the exchange rate.
Reading crypto balances
A portfolio screen may show token quantities alongside their estimated value in dollars, euros or another currency. Changing that display currency changes the measurement, not the assets held. Estimates also depend on the selected prices and calculation time.
Stability and scope
A widely used, relatively stable unit simplifies budgets, contracts and comparisons. Volatility makes future purchasing power harder to assess, although a volatile asset can still be used as a unit within a particular system. Being divisible into small units does not, by itself, establish broad use for pricing.