MDX Implementing Logical OR On Members of Different Hierarchies

In MDX, when two or more members of the same set are placed on a slicer, OR logic is automatically applied. For example, if you have two members (e.g. sizes 'Small' and 'Large') on a slicer, the resulting dataset will contain data where the first, second or both members occurred in the associated fact table. Similarly, a tuple formed by members of multiple hierarchies (e.g. size 'Small' and color 'Red') will only return results that have data associated to both members; AND logic is applied in this instance.