While in Floorplan Mode, you may add Boundaries to define a rectangular section of space for a particular purpose. This is often useful when you know you have, say, 250 square feet to arrange your fixtures within, and no more.
Add a boundary by either Right Click -> Add Boundary or Insert -> Boundary while in floorplan mode. The boundary will by default be unmovable. You can double click it any time and edit it to make it movable, to redefine is dimensions, or to change its color.