I was reading a journal based on SiC reconstructions, and they seem to mention that every dangling bond in the $(1\times 1)$ unreconstructed Si face or the $(0001)$ direction bonds up with a Si adatom placed in the corresponding $T_4$ position and hence leads to a $(\sqrt{3}\times \sqrt{3})R30^o$ reconsruction. 
Now, I do realize what is happening extremely clearly, but I want to know which nomenclature they use to name the position of the Si adatom on the C atoms (dark red dots) as the $T_4$ position. I would be really grateful for the help, any books or article citations in that direction would also help. I absolutely do not know whether it is a symmetry group or something else.
