Probably a dumb question, I know, but it's not a term I normally use. I ask because I got some Zipfit liners fitted the other day to replace the packed out liners in my Head Raptor 115 RS boots. The boot fitter observed that width-wise the 98mm last was OK for my rather narrow, skinny feet, and length-wise the shell fit was OK (probably a little under 15mm, slightly less than I had thought, which explains why the next size down was unworkable for me). My main problem issue was actually to do with instep, by which he meant, I think, that my foot was too 'shallow' for most boots to provide a snug fit - there would be likely to be a little room above my foot unless I buckled down very tightly (which is indeed my experience).
I haven't had a chance to ski with the Zipfits yet but I hope they address this problem. If they don't fully address it, what recommendations are there for boots which are likely to provide a snug fit for a 'shallow' foot (I'm sure there's a better word than that)?