Editing Reports (section)

=== What are filter-only dimensions and what are they used for (i.e., cross join)? ===
Filter-only dimensions can only be added to reports as Filters. More specifically, they let you create reports that cross join data across multiple areas of Custom Reporting (also known as a cross join).

For example, if you wanted to create a list of email addresses (Contacts) who have opened an email (Campaign) within a specific date range, you are referencing two different tables: Contacts and Campaigns. Adding a Filter-only dimension of Campaign Relationship Type will allow you to select the type of relationship that the Contact has with a given Campaign.

In this case, you would add a Filter of Campaign Relationship Type = Opens and a Filter for your Event Date. Then you would add email address as a dimension and run your report. This would give you a list of email addresses that have opened an email within the date range you filtered by.