Gluten
Responsible for the dough's framework: holding gas, volume, the ability to keep shape. The working threshold for graded flour is from 28%. Below that the dough weakens and spreads; noticeably above it, the dough turns springy and needs a longer proof.
Ash
Shows how much of the grain's outer layers remain in the flour. The lower the ash, the whiter the flour and the «cleaner» the taste. First grade — up to 0.75%, premium — up to 0.55%, Extra — up to 0.45%. High ash is not a flaw: it brings colour and flavour, but it changes how the dough behaves.
Moisture
The norm is up to 14.5%. Flour with raised moisture keeps less well and loses its figures faster. It is the first thing to look at when a batch «suddenly» behaves differently after a month in the store.
Protein
Indirectly related to gluten, but not the same thing. It matters for recipes with a long fermentation: at 12–13% the dough holds its structure longer.
How to use this
Write down the figures of the batch that gave you your best results and make them your internal standard. Then require a specification with the batch number for every shipment — and compare. That is cheaper than rebuilding the recipe each time.