I've been staring at this riddle for half an hour now and it's doing my head in.

I briefly considered a possible relationship to mako reactors, thinking there were several scattered around the world. But the presence of 8 in Midgar debunks that.

Then I thought about ultimate weapons. There are 8 materia slots, linked in couples. All of them are like this except Aeris's, which has 6 linked slots. Dunno if this could be on the right track, but I feel not.

Btw BG don't respond to these musings I'm just putting things down in case they help people.

EDIT: and in that half an hour Choco Boy has posted a brilliant guess.