A common scenario may be that a Space at your facility may be a multi-purposed room with multiple dividers that will allow you to make one big room into smaller individual rooms. 

Let's look at the following example, a big Space that can be divided into TWO smaller Spaces: 

Now f the entire SPACE is scheduled, we need to also check conflicts against the smaller Room A and Room B to make sure that there is not something scheduled in one of the smaller rooms. 

To do this, we need to setup the COMBO rooms.  
Let's go to Settings>Basic>Spaces and EDIT Room A/B (Entire Room).

Under the COMBOS area, we need to check ROOM A and ROOM B. This tells the system to also check conflicts for these two rooms any time the Entire Room is selected.  

Now, if we look at the Combos for Room A only, we would only want to check against the "big" room: