Hi Steve,
Thats a "Join" with those 3 surfaces...

A boolean is geared towards having 2 solids that intersect somehow and creating a new volume by calculating the new intersections and removing the unwanted pieces. The Join is how various surfaces are "Glued together". If you have 2 surfaces with a shared edge, then they can be joined at that edge. With proper construction of surfaces, having all edges joined into a closed shape, creates a solid.