Didn't Colje answer those questions in the front page already?
and right, HD stands for high definition meaning high resolution no?
Nowadays many subbers do start to release high resolution files like 1280×720 (720p) -->a common resolution for HD
Besides, what Colje meant is probably that when a group releases multiple releases and there are choices between SD and HD (either one of them are in h.264 anyway; most of them anyway), then BU will choose HD over SD release
now if a subber still release in xvid/.avi or SD files alone, which is getting a little bit rare nowadays, then there's no choice but to use that release is there?
If you're so worried about not getting the type of files you want then why not just go to other sites that provide both releases like tokyotosho?