How the grade works
Plans earn points on two measures, each judged against all plans in the same asset-size bracket for the same year: the reported administrative expense ratio (cheaper than peers scores higher) and employer contributions per active participant (more generous scores higher). Top-quartile on both is an A; bottom-quartile on both is a D. Plans missing either measure, which is common where costs are paid through fund revenue-sharing, get no grade rather than a guess.
What it does not capture
Fund expense ratios, usually your largest cost, are not in the public structured data, and neither is the match formula or vesting schedule. Treat the grade as a peer comparison on the two things the public record measures consistently, not a verdict on everything. Details: methodology.