Horizon is how many days an event must age before it may be resolved at all, and it is the whole of the no-leakage rule. 120 for the payment lane (past the Visa and Mastercard dispute windows), 14 for signup abuse. Unstated takes 120.
Now moves the observation instant BACKWARDS, RFC 3339. It exists so a BACKTEST can resolve labels as the plane stood at a past moment; without it, every backtest would score a model against knowledge that arrived after the decision it is being scored on. An instant after the server clock is refused: a backtest resolves the past, and a future one would declare unmatured events matured and hand a training set negatives for rows whose chargeback has not had time to arrive.
Subjects are the exact events being judged. Each carries its own event time, because the as-of that keeps the future out is derived from that instant plus the horizon — one as-of over a whole batch would give a January row six extra months of hindsight. One entry per DISTINCT (kind, subject, at): naming an event twice answers once, because an event resolved twice would list its own winner as a contrary claim and would hand a materialiser duplicate training rows.
