I like the post. When i first started diving int he 60's the general rule as told was that the 72's had a pluss rating for the first 5 years and it was lost at the 5 year point. Your post seems to explain how/why that position was taken back then.
Its a maximum allowable expansion in mL (cubic cm)
My lp cylinders have REEs on them, < than REE = pass for a +
My bank bottles all have REEs on them too, some of those are lp cylinders eligible for +s some are 4500psi bank bottles which aren't.
All my 3442 and 3500 psi tanks have REEs on them too, that is the max allowable expansion in CCs. If they are > the REE they fail
Basically the REE is a mega cheat so the hydro shop doesn't have to do a lick of math at all. < REE = pass (and + eligible if manufactured under 3AA specs)
If a 3AA tank is over the REE it can potentially still pass under the 90% rule. This is actually pretty rare. LP72s in hard to plus rate without the REE because you need the wall thickness and a whole bunch of other variables, plus math and more math. Hence they are typically only tested against the 90% rule which doesn't mean they aren't good to 2450...
AL3000 tanks are only evaluated under the 90% rule.