Definition
C6 Appendix A: “What we call a communicative object is an equivalence class of situations relative to a chosen family of measurements, or a compatible constraint family; it is not necessarily a single hidden vector.” Manuscript §3: “An observational equivalence class identifies realizations indistinguishable by the chosen procedures.”
What it is not
Not the same as TERM-002. An equivalence class partitions a given set of situations, which is the shared ambient space that A78 said the intersection picture (TERM-022) presupposed and the local-section picture does not. C6 put the two readings in one sentence joined by “or”, and the change was not remarked (the chunk-5 reader’s point). The two coincide when a global realization exists; when none does, only the constraint family is defined. A81 §3.4 had it right: “Define an object by its observational profile where a compatible global realization exists. Where no such realization exists, retain the family of local constraints as the primary empirical object rather than inventing a global state.”
Where it went
A203 §14 made observational equivalence “relative to the chosen probes” (CLAIM-142). A218 Ch7 went further: “A structural kind may be real while remaining indistinguishable under an insufficient family of empirical probes”. The equivalence class stops being the identity criterion and becomes a limit on knowing a kind (TERM-040). No turn addresses this entry.