When you add a field to a Query, there are three Usage modes:
- Full (default)
- In this query / criteria
- Criteria only
This means you have full usage of the field: it will visibly appear if used in Views based on the Query, can be used in the query and for criteria.
In this query / criteria
This means the field will not be shown, but it can still be used in this query, and be used for criteria in view embeds.
This means the field can only be used for specifying criteria in view embeds, and is otherwise not available.
For optimum speed of page delivery, you should set fields to "Criteria only" unless they need more usage.
Helping with sorting
If you have a Table with field "Firstname" and "Surname", and you want to display the record as "Firstname, Surname", but order by Surname, and then by Firstname, you would add fields in this order:
Surname (sort ascending, "In this query / criteria")
Firstname (sort ascending, "Full")
Surname (sort ascending, "Full")
Helping with links
If you have a Table with a Name field on it, which links to a destination specified in another field, Destination, you want to show the Name field, but not the destination:
Name (on-click link to Field: Destination, "Full")
Destination ("In this query / criteria")