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What counts as one face
A cylinder with Radial Segments 8 and Height Segments 3 is 8 faces if you are colouring it and 24 if you are selecting on it. Both answers are right, for different jobs, which is why Set Faces exists: it declares which one the rest of the chain should use.
Set Faces with A Face Is set to Quad treats each four-sided cell as one face. Leave it out and a subdivided flat side counts as a single face, so an operation meant for one cell takes the whole slab.
As builtSet Faces first
The declaration survives downstream. Inset and Extrude Tiles renumber as they add geometry, keeping the original faces in order, so a selection made before them still points at the same faces afterwards.