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-evidenced form (bioavailability or clinical outcome), Tier 2 is acceptable, and Tier 3 is a low-cost, low-bioavailability form. "Unrated" means we found no comparative study for that form, or the label names no form and the compound's default form carries no rating.
Example
Magnesium citrate is Tier 1 (it produced the highest serum and salivary magnesium response in a head-to-head trial); magnesium oxide is Tier 3 (virtually insoluble, ~43% soluble even under peak gastric acid).