Any work with unit economics starts with defining what a scaling unit is. It is the economics of such a unit that unit economics calculates. 

Historically, a unit was a commodity or a single transaction. And this approach is still used today, for example, when trading on marketplaces. 

But this approach has a significant disadvantage, the scaling unit is a commodity that does not sell itself, and you need a buyer and the process of getting this buyer.

And since, so, the modern approach to unit economics defines a scaling unit as a potential customer, which a business attracts with the help of marketing and then turns it into a customer who makes transactions, during its lifetime.  

This approach is universal and allows unit economics to be used for any business that scales sales by increasing the number of potential customers.

Take the choice of scaling unit economics very seriously, it will determine how you calculate unit economics and how you make decisions based on it.