The percentage on a bar is the share of it, by weight, that came from the cocoa bean: cocoa mass and any added cocoa butter together. The rest is sugar and, in a milk bar, milk.
So 72 per cent means 28 per cent sugar. It does not say whether the beans were good, how they were roasted, or whether cocoa butter was added to soften the texture. Two bars at 72 per cent can taste nothing alike.
A percentage is a recipe, not a review.
A higher number means less sugar and, usually, more bitterness. But bitterness comes mostly from fermentation and roast, not from the number. A well-fermented bean at 85 per cent can be gentler than a poorly handled one at 65.
Milk chocolate carries a second figure that matters just as much and is rarely printed: the share of milk solids. That is what decides whether a milk bar tastes creamy or merely sweet.
If you want to compare two bars fairly, compare the same percentage from different makers, and taste them in the same sitting at room temperature. That is the only honest test, and it is a pleasant one.