For a multi-value Tagbox (or multi-value Relationship, multi-value Fieldset), it is used in combination with the Nested Filter.
The Terms Relation Aggregator is used to aggregate terms using a unique ID as a base, showing the translation (from Display Pattern) in the Filter. The benefit is that IDs are always only used ones. Text e.g. List Item name "Joe Berrymore" may be present multiple times (see Terms Filter), so filtering for Joe Berrymore may bring the wrong results, whereas "Joe Berrymore" has a unique refId.
If you want to have a filter for the List "Persons" use the refId of the tags, which avoids any mix-ups with other tags. 
Picturepark documentTypes are "Content", "ListItem", "Schema", "User", "ContentPermissionSet", "Owner".
Filter Examples:
Provide the userId and get the user name.
Provide the contentPermissionSetId and get the permission set name.
Provide the contentId and get the Content Item name display pattern.
Provide the refId and get the list item name pattern.
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