Collision Filtering - Learning Game Physics with Bullet Physics and OpenGL - page 122

Note that the default values for group and mask are set to -1 in the declaration of
the CreateGameObject() function. If we remember our signed-integer represent-
ations, a value of -1 means every bit is set to 1. Thus, the default values for group
and mask make the object a member of every group, and has a mask enabled for
every group. This is the reason why the ground plane and our shootable boxes are
able to collide with both the boxes and spheres.