Quality tier
Our own 1-3 rating (not an official grading system) for how well a specific chemical form of a compound is supported by absorption or efficacy research relative to other forms of the same compound. Tier 1 is the best-supported form seen in head-to-head research; Tier 3 is the weakest, most poorly-absorbed, or entirely unstudied form. "Unrated" means the label doesn't specify a form precisely enough to rate at all.
Example
Magnesium citrate is Tier 1 (produced the highest urinary/salivary magnesium response in a head-to-head trial); magnesium oxide is Tier 3 (virtually insoluble, ~43% soluble even under peak gastric acid).