What is the difference between digest and extended abstract?
Conference format varies. Some conferences ask for more than just a short (up to one page) abstract, but something like a short paper containing some of the preliminary results. Some ask for a short abstract, screen the abstract, and ask for a full paper which will then be reviewed before the conference starts. Some others ask for an abstract and only ask for the full paper once the conference is concluded. I believe this is more of a tradition of that particular conference rather than the preference of the chairmen or reviewers.

It is important not to exceed the page limit, but make sure that you are not way below the limit.