TS 28.554

If 28.552 defines the raw counters, 28.554 defines what you do with them — the end-to-end KPI definitions and formulas that aggregate those measurements into the higher-level numbers people report on. Accessibility, retainability, mobility, integrity and the like: this is where 3GPP writes down how each is supposed to be computed from the underlying counters, so a "5G accessibility KPI" means a consistent thing rather than whatever each vendor decided.

Why it matters in practice: most KPI disputes are really formula disputes. When two systems report different accessibility for the same traffic, the cause is usually different formulas or different counter mappings underneath, not a different network. Having a standardized definition in 28.554 is what makes cross-vendor and cross-operator comparison meaningful at all. The everyday workflow rarely involves opening it, but when you're validating a KPI definition, building a custom dashboard, or arguing about why a number looks wrong, 28.554 (sitting on top of 28.552) is the authority you reach for.