I am just getting back into using Crystal Reports again after a long absense and several version changes. Now using the .NET version 10.5 in VS 2008.
The CssClass is .NET's reference to the css class that will be applied when the page/report is rendered. In order to get it to work correctly, you have to add the <link rel="stylesheet".... tag for your stylesheet either in your master page or in your standalone page <head section. If you look at the page source after rendering, you will see those items are converted into <span, or <div tags with the specifed CssClass (class="") references.
After much trial and error, I found the "Keep together" option does work, but must be appied to the group in order to span pages correctly when the group, or subreport content is more than will fit on the current page in it's entirety. The subreport has to contain a group in order to apply the keep together option.
The UnderLay Section option is for adding maps, watermarks, etc. that you want other content to print/display over that content. I have not used it, and I believe someone else already addressed this part of your question.
hth, Duane