Thank you for the reply!
It seems motherboards with a 5 in its SKU is target at mainstream users, while motherboards with a 7 in it SKU is target at overclockers. One major difference between these two board, is the amount of physical features a users can see immediately just by looking at the board.
A example would be
GA-X79-UD5 vs
GA-X79-UD7. The UD5 have 8 ram slot and UD7 has 4. UD5 has 1x firewire, 1x eSATA, and 1x eSATA/USB combo. UD7 has none of it.
Shouldn't a motherboard with a higher SKU number have more stuffs, than a board with a lower number in its SKU?