This record contains a short preprint introducing an operational notion of cosmological horizon for finite-lived observers in FLRW cosmology. Instead of assuming an ideal observer existing indefinitely, the paper defines an observer’s accessible region as the causal past of a finite worldline segment (the observer’s active lifetime), and relates this “operational horizon” to standard particle/event horizon concepts. The manuscript also extends the construction to composite observers (multi-agent or distributed instrument networks) and gives simple composition rules—union, intersection, and communication-limited fusion—that quantify how information access changes when multiple agents share data with finite latency. An optional appendix briefly mentions neuroscientific motivation for treating “the observer” as a system with constrained internal integration, but the main results are purely causal/relativistic and theory-minimal.This note is a self-contained stub/excerpt motivated by a broader project (“C-Theory”). It does not assume or require the rest of that framework.