When you are combining two or more dimensions, or two or more attributes from the same dimension, to design a custom hierarchy in a crosstab, you may want to useMultidimensional Subtotals.
E.g. if you have Countries as the first level dimension and Store Types as the second level dimension, then you may have the same Store Type represented across all countries. While TARGIT perfectly displays subtotals for the Countries (because they are on the top level), you may also want to know the subtotals for the Store Types - i.e. the sum of each Store Type across all Countries. For this, you will need to enable Multidimensional Subtotals.
Once enabled, you will also be able to refer to the Multidimensional Subtotals in TARGIT calculations - opening up for even more usefulness.