With a 6x12 back you would focus on the groundglass, remove the groundglass, clip in the back with the Graflok slides, and shoot.

With the 6x17 back there is a separate viewer for focusing (since it is wider than 4x5" and the film plane is set back from the normal film plane) that you would use in place of the normal groundglass, and you would switch it with the rollfilm holder to make the exposure. The 6x17 back also includes masks for 6x9 and 6x12, but the whole arrangement is bulkier than a 6x12 back, not only because it's bigger, but because you need the separate viewer. Also, you can't switch between formats on the fly with any of these backs.