TS 28.552

28.552 is the 3GPP spec that defines the performance measurements — the raw counters — collected from individual network functions and network elements. If you've ever wondered where the canonical definition of a given 5G counter is supposed to live (what exactly it counts, on which NF, under what trigger), this is the document. It covers measurements across the 5G system: gNB/RAN side, the core NFs (AMF, SMF, UPF and the rest), and so on.

In the standards stack, think of 28.552 as the measurement layer and TS 28.554 as the KPI layer on top — 28.552 says "here is the counter and what it counts," 28.554 says "here is how you combine counters into an end-to-end KPI." You won't sit and read it cover to cover, but it's the reference you cite when a vendor's counter behaviour looks off, or when you're trying to compare two implementations and need to check whether they're even counting the same event the same way. Real products often add vendor-specific counters beyond the standard set, so the spec is the baseline, not the whole story.