What does this equation mean?

Glickman’s RD is this same object, derived from a different starting assumption but converging on an identical use. He states the correspondence in his own words, not as an analogy this article is imposing on his work: a Glicko rating and RD together are “usually more informative to summarize a player’s strength in the form of an interval… report a 95% confidence interval. The lowest value in the interval is the player’s rating minus twice the RD, and the highest value is the player’s rating plus twice the RD” [ 3 ] . Line up the two constructions and the correspondence is not loose: $R$. ​ Rating deviation $\mathrm{RD}$ , bounded to Showdown’s 25–130 range 95% interval: X $\pm$ 1.96 $\cdot$ $\mathrm{SEM}$ 95% interval: R $\pm$ 2 $\cdot$ $\mathrm{RD}$ [ 3 ] Reliability rises with more independent items RD falls with more games played in a period [ 2 ] Reliability falls with time since calibration RD rises with time since the player’s last game [ 3 ] This is the article’s original contribution: not the observation that a rating has “uncertainty,” which anyone who has seen a percentage sign after a GXE could guess, but the derivation that Showdown’s published RD update schedule is doing formally, on Glickman’s own stated terms, exactly the job Novick’s 1966 axioms assign to a standard error of measurement — no metaphor, no rounding of two different ideas into looking alike, one construction restated in the other’s notation.