Simon,
I guess from the lack response, not many people are aware of Rule 41..
Its similar to a certificate of conformity for corrugated packaging used in the US.
Effectively you print a stamp on the bottom of a case details such as Burst Strength, Combined weight of Facings and ECT strength.
Depending on the strength properties a case is then certified to go upto a certain size with a certain weight of product in it.
If you export to the states, it is likely that you have a circular printed code with this information.
I think this certificate also reduces your shipping insurance costs so is beneficial for US companies.
Sites like DHL do have some basic information but I'm after a full formal reference document
Regards
Rich