This is a specialized control in the designer for auto-incrementing a field. This control is based on a text box and will automatically increment by one its value based on the highest previous value in the ACT! database. Any time you create a new item the value will auto-increment. If you open an existing item that was previously left blank then the value will auto-increment.
Follow these steps to add the auto-increment control onto the form.
Open the form in the Designer
Click on the Specialized Controls button in the toolbar.
Drag the item labeled Auto Increment onto the form
A Select field window will come forward choose the table name