Measures and Measure Groups - Expert Cube Development with SSAS Multidimensional Models - page 101

It's usually best to keep IgnoreUnrelatedDimensions set to True since if the users
are querying measures from multiple measure groups, then they don't want some of
their selected measures suddenly returning null if they slice by a dimension that has
a regular relationship with their other selected measures.
Handling different granularities
Even when measure groups share the same dimensions, they may not share the same
granularity. For example, we may hold sales information in one fact table down to
the day level, but also hold sales quotas in another fact table at the quarter level. If
we created measure groups from both these fact tables, then they would both have
regular relationships with our Time dimension but at different granularities.
Normally, when you create a regular relationship between a dimension and a
measure group, Analysis Services will join the columns specified in the KeyColumns
property of the key attribute of the dimension with the appropriate foreign key
columns of the fact table (note that during processing, Analysis Services won't
usually do the join in SQL, it does it internally). However, when you have a fact table
of a higher granularity, you need to change the Granularity attribute property of
the relationship to choose the attribute from the dimension you do want to join on
instead, as shown in the following screenshot: