What 99.7% aloe vera actually means on a label

It means that of everything that went into the bottle, 99.7% by weight was aloe vera inner leaf. The remaining 0.3% is the stabiliser and preservative system, and the number is printed there because Australian food law requires it, not because the brand chose to be generous.
​That second part is what most people miss, and it changes how much credit a high number actually deserves.

Why the number is on the pack at all

Because Standard 1.2.10 of the Australia New Zealand Food Standards Code requires it. Where an ingredient is named in the product name, or a buyer would reasonably associate it with the product, the percentage of that ingredient has to be declared. Aloe vera in something called Aloe Vera Gel is about as characterising as an ingredient gets.
​There is no minimum threshold. A drink with 2% aloe has to print 2% just as plainly as one with 99.7% does. That is the useful part: the number is not a boast the brand opted into, it is a disclosure it could not avoid.

What the number does not tell you

Almost everything that matters. The percentage is a quantity, not a quality, and four things it stays silent on are the four things worth knowing.
It does not say which part of the leaf you are drinking. Inner leaf and whole leaf can both be 99.7%. It does not say whether the aloe was pressed from fresh leaf or reconstituted from powder, and both can be 99.7%. It does not say what the aloin level is. And it does not say how long the leaf sat between cutting and processing, which is most of what separates one bottle from another.
​Two products can print the same number and be meaningfully different products. That is not a loophole, it is just the limit of what one figure can carry.

The check, in about ten seconds

Turn the bottle around and read three things in order. First, the percentage. Second, where aloe sits in the ingredients list, because ingredients are listed in descending order by weight and the first one is the largest part of the bottle. Third, whether the pack says inner leaf or whole leaf anywhere at all.
If the percentage is high but the pack will not tell you which part of the leaf it came from, the number is doing public relations work rather than informing you. A brand confident about its processing says so.
​For what it is worth on my own range: Aloe Vera Gel is 99.7% inner leaf, Berry Nectar 91%, Mango 85.8% and Peaches 84%, with the difference going to fruit concentrate. I earn a margin when you buy through my link, which is exactly why the method above is written to work on any brand's label, including the ones I have no relationship with.