Field Identifiers are used to create mass mailings & documents which are personalised to each individual/group.

When you select one of the Field Identifier options, e.g. Customer File, the following screen will be displayed.

Every field within the Customer File has been replaced by a tickbox.  If you want to insert the field identifier for the Surname, select the box as in the example below.

Select F3 Accept and you will be returned to your document.  The relevant field identifier will have been inserted at the point you selected, e.g. @MB#SUR.