User scale is a thing that acts as a Multiplier to the real life height, but this is a concept that brings in a lot more complex topics than a user may think.
Usually using user scale is an alright solution with permissions, and works in 99% of cases, but for those who want to slip through the cracks there are more extreme methods explained in the rest of this page that will allow for more fine tuned control for puppeting systems like 989onan's MMD player.
How real life height has implications with user scale
How user scale first works first starts with how tall you are in real life. You may be 4 foot tall, or 8 feet tall. There are many extremes to how tall a user may be. There are also tools for steamVR and occulus that can fake this, allowing for a user to fake the size of themselves and their playspace, making them seem only 1 inch tall when they are actually massive. Resonite then takes whatever data is fed in and multiplies their size by their user scale.
This means when it comes to user scale, the actual size a user may be in a world setting may actually be a lot smaller or bigger than their avatar scale may first imply. They can be 1 user scale but actually be 12 feet tall, making avatar scale useless in these situations.
Now one may be inclined to use a bounding box, to get those unruly people from becoming too tall and ruining a small people's world. But there are problems with this too. A user's lasers, or a user crouching can make the notion of using a bounding box go out the window. Someone could point their laser straight up and obliterate themselves in an instant due to floating point error when you try to scale them to the right size according to bounding box size.
The ""correct"" solution for user size
Avatars can be big, small, and their bounding box doesn't mean a thing. How do we prevent users from getting through small gaps or becoming massive and destroying map intentions?
This is where spine measuring comes in. It has it's caveats, but can solve the problem. First to note, an avatar's spine is very very plastic. They can be missing a chest, spine, and neck bone and still function. They can even be missing a VRIK (Which we can't really solve yet). But with what ProtoFlux tools we have, we can try to fight back. you can measure each body-node assigned bone in global space, using the distance between each one, and add them together excluding null ones to get the length of their spine. Then adding their legs you can get an extremely rough estimate on how tall they are. This can be off by 1-2 feet, but is usually foolproof for a user with VRIK. This only works if you use bones that have body node assignments, and ignoring bones in-between since those can be used to fake local transforms positions of bones and throw off your measurements.
Once an avatar has been measured, you can scale them down to stay within a specified range, and that will stop them from going way out of range. This doesn't always work, but it's the closest we got to a foolproof system