By selecting this drop down the toolbar just below the drop down will display a list of generic control you can place on your data-entry form.
Generic controls are defined as controls which have not been attached to a specific field in the custom table. Once you drag one of these generic fields you will be presented with a dial to select the field from the custom table.
You can use these controls to place two controls attached to the same field. This may be handle if you want to see a field under two different tab or in two sections of the data-entry form. These two controls will sync with each otehr so when the value of one control changes the value of the second control is updated.