Kinda right, but not quite.
Fem seeds aka feminized seeds, are the result of induced pollen from a true female pollinating another female clone, be it a genetic clone copy of the plant that was induced to produce pollen, or a genetically different female plant.
In the case of pollen fertilizing a gentically identical clone, the resulting seeds are labelled as S1. If the pollen fertilizes a genetically different clone, the resulting seeds are hybrid.
S1's are just a type of feminized seeds, a subdivision of kind.
-Chimera