Again, from the SAP Help - two important points to
remember:
1. In a multi-planning area, you can only define selection conditions for characteristics which occur in
all basic planning areas of the multi-planning area. These conditions are used when reading data in all involved basic
planning areas. Key figures and characteristic values, which are not contained in all basic planning areas, are included in
the multi-planning area with the initial value.
If values should be generated for characteristics or key figures by a
planning function, which is not available in a basic planning area, these values are ignored when saving.
2. You
can change the composition of the basic planning areas that make up a multi-planning area until the multi-planning area is
saved in the data basis. After saving, you can make no further changes. This is similar to the restriction with normal
planning areas. You cannot subsequently change their assignment to an InfoCube.
(courtesy: SAP Help)