Wall Split System are designed to individual heat or call a room,
the wall mounted models are designed to sit high on the wall below the ceiling and
remain very unobtrusive and the outdoor machine is then generally installed behind the wall unit or in the vicinity.  
Multiple machines can be installed to air condition the whole home and give you the ability to choose
which rooms you want to heat or cool at any one time. 
Multi Wall Split systems consist of one outdoor unit running multiple indoor units.

They are good for situations where their is limited space outside for more than one unit.
Or if you just don't like the look of too many air conditioners outside.
They are genarally more expensive to buy and install than seperate units due to the fact
that the connecting pipes and electrical controls have to run back to one central place.
With seperate units, pipe runs are generally much shorter as the indoor and
outdoor air conditioning units are place as close to each other as possible.