The actual magnesium content of a compound, ignoring the carrier salt.
Magnesium is sold as a salt of an organic acid — glycinate, citrate, malate, oxide. The "1000 mg magnesium glycinate" on the label is the whole compound weight, not the magnesium itself. Of that 1000 mg, roughly 14% is elemental magnesium (140 mg).
Always check the supplement-facts panel for the elemental magnesium number. It's the figure to compare against the RDA.
Same principle applies to iron (e.g. iron bisglycinate is ~20% elemental iron) and zinc.