Ultimately, you want an even and very smooth surface.
This means that every square millimeter is sanded so thoroughly with each grit used that the sanding marks of the previous coarser grit disappear in each case. So it is a big mistake if a certain grit could not cleanly sand over the last grit. Such a spot cannot be smoothed with subsequent finer abrasives. In the end, the paint here will also not show a smooth surface.
Here is a list of what the possible prepared areas may look like: