The basic difference between Open and Open add space boundary conditions in CST

Hii
Anyopne tell me what is the basic difference between Open and Open add space boundary conditions in CST Microwave Studio?
When i use open boundary condition then i m getting different result in comaprison to Open add space boundary condition.
i don't understand why such things happen n what is the basic problem within that?
plz reply..............

Hi,
in the help is:
Open (PML): Operates like free space: waves can pass this boundary with minimal reflections. Note that in case of a unit cell simulation with the general purpose frequency domain solver open boundaries are realized by a Floquet port.

Open (add space): Same as Open (PML), but adds some extra space for farfield calculation. This option is recommended for antenna problems.