Question Please tell me about height limits regarding carseats?

So I know some carseats go by the 1" rule or a standing height limit. Then I have also heard some people will make a parenting decision to disregard the standing limit. Why do carseats have these limits? What's the different between a leggy or torso-y child? How is this number arrived at? Does it involve physics and complicated calculations, related to the size of the dummies, or is it an arbitrary number?

TIA

Rear facing you have the 1" rule, generally. Some seat are head even, some are 1.5".

We're not sure how they get to the numbers. The three year old dummy is 37.2". So Evenflo's makes sense (37" rear facing). Some Dorel seats are 40", some are 36". No idea how they come up with those. They also say for some seats that it's 40" or head even with the top of the shell (or 36"), or sometimes it's 1" or the height limit. We have no idea why legs matter. I've asked many people that and never been told any answer at all, nevermind a good answer. Hopefully someone else will know, because I don't.

Some seats don't have a standing rear facing height limit. They just go by a relative height. It's not required to have one for rear facing (on a convertible).

Forward facing there has to be some limit, though for combination seats it may be harnessed and boostered, or just boostered. My guess with these is they maybe make a guess on torso height proportionate to overall height? Or they throw a dart. Seriously, either way. Some seats have VERY inflated height limits, others are a bit more accurate.

FMVSS213 requires a height label. But it doesn't require one for every mode. So an infant seat will always have a stated height because it's required and there's only one mode. You then need to ask the company which they prefer. Overall or relative height. Some care a lot more than others. For a convertible it may say 49" height limit, and that's rear facing or forward facing, but there's another limit for rear facing (1") that's hit first. Other seats specify a forward facing overall height limit, but not a rear facing one. For combination seats you have a height listed, but it's often the booster overall height, not necessarily a harnessed one. The Radian went from 53" as a Sunshine Kids Radian to 57" as a Diono Radian with booster, even though the top slots didn't change heights, and most 4'9" kids (57") wouldn't fit in it with a shoehorn and a vat of grease. As a harnessed seat or boostered.

But that's why there's an overall limit. It's required. But many manufacturers will admit that the overall height is more important, some will require both the standing and relative heights to be met. And some change their minds (looking at you, Graco and Evenflo) and have some seats with some and some seats the other way.

Wendy