What exactly does the reaction direction mean

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See https://biocyc.org/PGDBConceptsGuide....:_tex2pagesec_4.2.

Short answer: left and right are assigned according to EC conventions (e.g., all hydrolases are written in the hydrolysis direction), which may not accurately reflect thermodynamic or physiological constraints. The value of the direction slot is intended to indicate the actual direction in biological contexts. So if your example reaction were listed as right-to-left, that would indicate that evidence suggests the reaction proceeds in the direction of electrolysis (which of course would be highly unlikely in this particular case). The purpose of doing things this way is to be consistent with EC conventions.