Whenever you select a usable or complete expression element using the drop-down lists, the Add button becomes available. This button displays the text to be added to the expression.
For example, if the following were chosen:
Dimensions from the Expression Element Type list
D1 from the ID list
X from the Extension list
Nom from the Second Extension list
Then the Add button becomes enabled and would have the following text: Add D1.X.NOM.
Upon clicking the Add button, the text then appears in the edit box at the bottom of the dialog box.
When you click the OK button, the text from the edit box is added to the Edit window, to the expression field where your cursor is. If you select an item from the Edit window's expression field, and the text to be added contains parenthesis, then the selected item would be placed inside of the parenthesis of the added text.