Are you talking about the action "Select in Field"?
The edit box allows you to enter expressions/variables/$-sign substitutions/search strings and everything else that leads to a specification of what exacly should be selected in the Field.
You can make selections in only one field with the Select in Field action. But you can select many values in that one field with the search string. You can use as many conditions in the search string as you need to return the values you want to select.
