When finishing the spot, the commercial should be edited in widescreen (16:9) staying in HD (not "downconverting" to Digibeta), and then output to an HDCam or D5 tape.
To use the same spot for the Standard Definition (SD) regular TV audience, there are several choices:
Downconvert the spot from the HD-finished version to a letter-boxed version on a Digibeta tape. This also allows advertisers to insert text or alter the black bars above the spot; for example, IBM "blues out" the bars or UPS "browns out" the bars.
Take the HD wide-screen spot and "edge crop" it, that is, cut off the left and right edges of the screen. You can prepare for this by "protecting" the 4:3 image while taping the commercial in HD, so the standard viewing audience will see the "protected" images in the center of the screen in 4:3, while the HD audience sees the full widescreen image. If you choose to "edge crop" the completed spot, graphics will also need to be in the 4:3 protected area.
Edit the HD spot by finishing it in 4:3, which will fill the screen for the traditional TV audience and then make additions in the post-edit session.