The current Room Boundary tool determines room boundaries at the intersection of walls/doors/windows/etc and floors, where they meet at level of the Room Calculation Point.
One routine problem is where bounding elements like walls do not meet precisely, due to irregular geometry or you have a large space broken up into zones without internal walls diving those spaces -- the Room Seperator line can be used to fill these gaps, (this creates ongoing issues with bounding elements moving but the Room Separator lines not being manually moved with them).
Another common issue is where the Room is located on a mezzanine level, or where the walls do not intersect the floor of the room -- (a solution is to create a Level for that mezzanine, along a Plan View, then place a new Room on that, go back to the main floor plan and Tag the new room), a cumbersome approach to an issue that confounds many users; part of the issue being that you cannot change the Room's Level constraint, nor in many cases the Base Offset. This leads to a large number of Rooms being orphaned or 'Not Placed', and Color Fills not working, (which in turn frustrated users often cover plans with a mess of Room Separator lines).
A tolerance setting between both horizontal gaps (ie. walls) and vertical gaps (ie. room calculation to floor/level) would immensely help solve these issues.