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.