In the sketch M8 6H means what?

M8 - metric thread 8mm
6H - tolerance for internal thread, 6H class - for General purpose
Please see the flowing links:
http://www.engineeringessentials.com/ege/fast/fast_page7.htm
http://www.calcool.dk/metricthread.htm
http://mdmetric.com/tech/M-thead%20600.htm
Please find in attachment the thread standard or check this link: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDkQFjAA&url=http%3A%2F%2Fwww.chinafastener.biz%2Fgb%2Fam%2Ffile_list.aspx%3Faction%3Ddownload%26ID%3D215%26ext%3Dpdf&ei=nHRmU4mZEoWm0QW-04GgCg&usg=AFQjCNF5PuOBzoh3rL9JEDXIoCjSrpvwvQ

m8 means metric thread 8mm of ISO standard and 6H refers to tolerance of threaded hole (know more about it in ISO 2768)

M8 means metric thread of 8mm outer diameter. but i dont have idea about the 6H

Metric fastener size designation nomenclature. As fully explained in ISO 965-1,