Since Smart Shapes are far more flexible (when it comes to formatting) than standard buttons or text captions, I'd love to see Adobe completely replace text captions and buttons with Smart Shapes down the road (perhaps Captivate 9... 10?). As it stands, standard text captions are used whenever you record a software simulation or enable feedback captions for interactive objects (you can use Hint, Success, and Failure captions for Click Boxes, Buttons, and Text Entry Boxes).
If you'd like to use Smart Shapes instead of Text Captions during the Software Simulation recording process, open Captivate's Preferences dialog box (Edit menu if you're using Windows; Adobe Captivate menu if you're using a Macintosh).
From the Recording group at the left side of the dialog box, select Modes. Finally, for each mode you'd like to use, enable Add Text Captions and Use Smart Shapes instead of Captions. You can select the type of Smart Shape you want to use via the Smart Shape Type drop-down menu.
When you record your next Software Simulation, the standard Text Captions will automatically be replaced with Smart Shapes.
As I mentioned above, you can also use Smart Shapes instead of standard captions when adding Hint, Success, and Failure captions. It's a simple set up (assuming you're using Adobe Captivate 8.1), reopen the Preferences dialog box. From the Category list, select Defaults. Lastly, from the General area, select Use Smart-Shapes for SFH Captions instead of Text Captions.
If you'd like to change the shape of the Smart Shape, it's a simple matter of selecting the shape and, on the Properties Inspector, using the Basic drop-down menu.