R group search

Could you explain what is meant by the "empty set" when performing an Undefined R group query?  For example, what structures should I expect to retrieve when selecting "Group or H or empty"?

This means that also the molecules which have no atom in the R-atom possition will be retrieved as an example of the atom with empty group is the middle carbon in H-C=C=C-H.

Excellent!  Thanks for the quick reply!