Suppose there are two polygons that are closer than 10m to each other. I would like to either combine those two polygons or create a new polygon that fills the empty area between them.
This rule requires that there are no voids within a single polygon or between adjacent polygons. All polygons must form a continuous surface. An error will always exist on the perimeter of the surface. You can either ignore this error or mark it as an exception. Use this rule on data that must completely cover an area. For example, soil polygons cannot include gaps or form voids—they must cover an entire area.
This cheat sheet shows all topology rules very clear.
This is my example layer with polygons (50 x 50 m), see distance between polygons: