MDX Union Set behavior?

[bonniehsueh]

All-
Why is the union() and nestedset() MDX functions able to union members at different levels in its own dimension, but not able to union across dimensions?

The error I get is: PCA-ERR-0024: The "unionSet" operator is only applicable on sets with the same dimensions. The number of dimensions in two or more sets is different

Believe its due to the behavior of working with an OLAP data source and not a limitation in the software, but would like a more detailed explanation.

Thoughts?