What is grouping?

Grouping means gathering into a comma separated list two or more selectors that share the same style or into a semicolon separated list two or more declarations that are attached to the same selector .
1. The selectors R, P with class name .first and class .footnote share the same style, e.g.:
R {font-style: san-serif}
P.first {font-style: verdana}
.footnote {font-style: italic}
To reduce the size of style sheets and also save some typing time they can all be grouped in one list.
R P.first, .footnote {font-style: serif}