At various times in history "unit of account" and "means of settlement" have been two different things. We are used to having a single currency (INR/USD etc.,) perform both the roles; however that needn't be the case. Going back to the earliest times in Mesapatomia, the debt and payments were denominated in silver coins but the actual payment was done in kind (such as food grains, wood).
Roman currency continued to be a widely used unit of account in the Middle Ages despite there being no Roman Empire producing the currencies/coins. Because a wide swathe of population had gotten used to that unit of account they just continued; though the payment was settled using whatever they had with them.
Brazil used this phenomenon to their advantage to escape from a monetary hole[1]. They created a new currency purely as a unit of account i.e., they didn't print/issue them. Everything was denominated in this new currency so citizens got used to calculating everything in this stable new currency. And then they declared it as their official currency.
I am fascinated by history of money and it's interesting to see a phenomenon play out again and again in different contexts.
[1] https://www.npr.org/sections/money/2010/10/04/130329523/how-...