Bitcoin is not a unit of account.
Nobody is keeping their business' books in Bitcoin. Not even those that accept Bitcoin. The Bitcoin block chain is not an accounting ledger for businesses.
Bitcoin is not a medium of exchange either by any measure.
When you go out to eat, the menu is priced in dollars. The meal is valued in dollars, not Bitcoins. When you say you want to pay with Bitcoins the restaurant calculates the equivalent based on the BTC/$ exchange rate.
In other words you have to sell your BTC. The POS is acting as an exchange but instead of giving you dollars for your Bitcoin
they give you a meal with that dollar equivalent value. You are still paying with dollars.
Nor is it a store of value given its unprecedented volatility. Oh and what would that value be measured in? Fiat!