When you apply a blending mode to an object, its colors blend with all objects beneath it. If you want to limit the blending to specific objects, you can group those objects and then apply the Isolate Blending option to the group. The Isolate Blending option confines the blending to within the group, preventing objects beneath the group from being affected. (It is useful for objects that have a blending mode other than Normal applied to them.)

Group (star and circle) with Isolate Blending option deselected (left) compared to selected (right)

It is important to understand that you apply the blending modes to the individual objects, but apply the Isolate Blending option to the group. The option isolates blending interactions within the group. It doesn’t affect blending modes applied directly to the group itself.