Whenever you select a usable or complete expression element from a list, the Add button becomes available. This button displays the text to be added to the expression.
For example, if you chose the following items for your expression:
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 has the following text: Add D1.X.NOM.
When you click the Add button, the text then appears in the edit box at the bottom of the dialog box.
When you click the OK button, PC-DMIS adds the text from the edit box to the expression field in the Edit window 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.