The primary data view controls the existence of entries in the join data view. The secondary data view provides supplementary data for this list of entries. In other words, if an entry exists in the secondary data view but not in the primary data view, it does not appear in the join data view.
The primary data view is the authoritative source by default. When an attribute is present on both source data views but has a different value on each, a multi-valued attribute is returned. This behavior is configurable, however. For example, you can choose to accept only the value in the primary data view, or only the value in the secondary data view.