This button shows current Form Template and could be used to change it. Clicking the button opens a popup displaying a list of available templates. Select template and click Ok.
Note that this action don´t save the ticket, only the user interface will be updated and the fields in the selected Form Template is displayed. To actually save the ticket with the new Form Template you have to use the Save button, or shortcut CTRL+ALT+U or CTRL+ALT+S.