It's the bar clamp size, MTB is 25.4mm and road 26ish, traditionally, of course both sorts now come in the oversize 31.8mm clamp.
All of my handlebars and stems have a 25.4mm clamp size.
However having just been out I may not need a 90mm, it's weird I'm trying to get the drop bars dialled in, I decided the short high wasn't working for me. But it's quite hard to identify what feels right, I have some neck/shoulder pain from being off the bike so long, my wrist is still healing and that's also putting more pressure on the problem left elbow, plus I've gained weight.
At some point I'll probably convert to 31.8mm oversize for all the bikes, but I want to get dialled in first. Regardless, next Friday is going to be tough for me, it'll be the longest by far sine May for me.
I think I'm good for now, I was able to move the stem from the Surly to the giant and then use an adjustable stem to get a reasonable fit on the Surly. I've been using the stem chart page to do the calcs but it gets confusing when dealing with differing TT lengths, HT angles and ST angles.
The Giant has sloping geometry and the effective top tube is slightly shorter than the Surly. However the Giant has a steeper seat tube angle, and shallower headtube angle, thus less of the top tube is rearwards of the BB. This means that, though the Surly is a much bigger looking bike, it actually needs a longer stem than the Giant to achieve the same reach from saddle to bars. Obviously this assumes the same relative position between saddle and BB on each bike.
I've tried to use a measurement from the BB to the HT, in the same way that you'd measure an effective top tube. Bloody hard to do with a straight edge, a spirit level and a tape measure, and only two hands But it does seem to show a reach from BB to HT on the Giant of about 10mm more than the Surly.
So, no, no stems required for now, but thanks for looking mate.