From Looney Pyramid Games Wiki

IceSheets are rule sheets for Looney Pyramid games that are in a new, consistent standard layout and format.

The purpose of this tag is to unite those games for which an IceSheet has been created. Some are already officially recognized rule sheets (vetted by the community and sanctioned by Looney Labs). Others were created because the designer prefer the format or would like to showcase their game within the community. We welcome feedback on the latter, as there is an approval process in place to get these games showcased on the official website.

Having these grouped may ease the process of evaluating the IceSheets and provide for a simple way to find the available IceSheets for those who enjoy collecting rule sheets in this new format.

More information is available under the initiative's page and here in this thread.