Paul Turley provides an explanation:
If there’s one concept that trips up nearly everyone moving into Fabric, it’s capacity units — because a CU isn’t what most people assume it is.
It’s compute, not seats.
A Capacity Unit has nothing to do with user counts or license tiers. It measures compute power, consumed in CU seconds as work actually runs. A report rendering, a pipeline moving data, a notebook chewing through a dataframe — all of it draws from the same shared pool of CU seconds on your capacity. Think of it less like a parking pass and more like a utility meter — it just keeps ticking based on what’s actually being computed, no matter who’s asking for it.
Read on for more information around CUs, as well as what F-SKUs provide and when you might want either.