Question

What is the difference between an open surface and a closed surface?

Answer 1

Open surface	Closed surface
An open surface has boundaries.	A closed surface is completely enclosed, meaning it has no boundaries.
Flux calculations through open surfaces consider the flow passing through their boundaries.	A closed surface is useful in Gauss’ theorem for calculating flux in vector fields.
Examples: A hemisphere, a disk, or a sheet of paper.	Examples: A sphere, a cube, or a closed cylinder.