When to select output format
On a prompt page? This is not a good time to select a different output format.
As you probably know from watching Running a Report in CU-Data, CU-Data's Cognos 11 software lets you switch output formats after your initial run. Also, not shown in the video, you can select an output format at the outset, by selecting run as from the three dots "more" menu that appears to the right of the report name.
But the Cognos 11 software also lets you select an output format when you are on an m-Fin prompt page. Don't do this (see reference Tower of Power). It bypasses some of the prompting, which can get really confusing.