Wrote up a little bit about this here: https://www.shapeoko.com/wiki/index.php/Carbide_Create_Basics#Boolean_Examples

For everyone’s convenience:

====Boolean Examples====

For selections involving only two objects, it is worth examining all the possibilities:

If the rectangle is the key object (indicated by its being drawn with a dashed line):

=====Boolean Intersection=====

(For this operation, which object is key does not matter.)

=====Boolean Subtraction (Rectangle as key)=====

=====Boolean Subtraction (Circle as key)=====

=====Union=====