How to Use Boolean Selections in COMSOL Multiphysics®
By using Boolean selections, you can create new selections of geometric entities in your model from other, previously defined selections. These include the Complement, Difference, Union, and Intersection selection types. These modeling tools create selections that are: the inverse of other selections; the entities in one selection, but not the other; the combination of two or more selections; and the entities that are contained in all input selections, respectively.
Adjacent selections are used to create selections of geometric entities that are adjacent to other selections.
Note that all of these selection types update automatically if you change your model's geometry so that you don't have to recreate the selection.