Five fields inside the notification decide the case.
A chargeback notification is not an email in any useful sense. It is a structured record in email format, written for processors and lawyers and only incidentally addressed to a merchant, and the disorientation merchants feel on first encounter is mostly a problem of not knowing what to look for. The standard format always includes the same five fields: a reason code, the transaction reference (ARN), the disputed amount, the response deadline, and a submission portal URL. Read those before reading anything else.
The reason code is the most important field, because it determines what the case is about and which evidence has any chance of winning. A 13.1 case turns on delivery records, a 10.4 case on cardholder-relationship evidence, an 11.3 case on authorization timing. The same documents do not transfer across the three. The deadline is the next thing to find, and it is the most often missed, because the email tends to express it in absolute date format from a processing date the message itself does not always make obvious.
The disputed amount and the ARN tell you which order is being argued and whether the full transaction or only part of it is in dispute. The submission portal is the dashboard your acquirer expects you to use; responding to the email itself, or submitting through Visa Resolve Online directly, will land too late or not at all. The acquirer is the funnel through which the response reaches the issuer, and the funnel is narrower than the email implies.