FEI Article 257.2.4
For all international Young Horses Competitions (five *, six, seven and eight year old Horses): All hind leg protections must have a maximum interior length of 16 centimetres; the width of the fastener must be at least five centimeters (refer to FEI Jumping Stewards’ Manual on the FEI website for diagram).
* NB: Competitions for five year old Horses may only be held at the FEI World Breeding Jumping Championships for Young Horses, unless special authorisation has been granted by the FEI.
The following criteria must be respected in relation to hind boots worn in international Young Horses Competitions (see also the FEI Jumping Stewards Manual on the FEI website):
The inside of the protection must be smooth. Only non-elastic Velcro-type fasteners are permitted; no hooks, buckles, clips or other methods of attaching the fasteners may be used;
The rounded rigid part of the protection must be placed around the inside of the fetlock;
No additional elements may be used in conjunction with the protection.
Here is a page from the Stewards Manual showing examples:
The gist of the rule is that hind boots mustn't have elastic, must have only wide velcro closures, and mustn't have any protrusions on the inside (smoothness refers to a lack of protrusions rather than to a specific type of material, as far as I am aware).
This rule, designed for the welfare of the horse, aims to reduce the practice of over-tightening boots to increase sensitivity and the use of boots designed to exaggerate the movement of the hind end over fences by applying pressure.
There are not many brands of boots that currently fit these criteria. I am currently aware only of the Young Jump boots made by Veredus that are definitely allowed and will update this post with any others that I come across. There are several basic neoprene types of boots that should fall within the guidelines, but it's best to check in person before purchasing to make sure that there is no elastic on the straps.