HLA-G shows comparatively limited polymorphism (16 protein alleles described) when compared to classical HLA loci. HLA-G is expressed along with HLA-C on the extravillous trophoblast. HLA-A, -B and DRB are not expressed on the extravillous trophoblast and primary trophoblast cells are HLA null. HLA-G and -C interact with the activating and inhibitory ligands of NK cells which are prevalent in the maternal circulation around the placenta. It is believed NK cells protect the placenta and also allow the development of the spiral arteries where the interaction between HLA-C and G the activating ligands of NK cells is important. I think restricted here is most likely to be a synonym for limited, rather than referring to NK cell restriction.