A story is a promise to talk.
A user story isn't a mini-specification. It's a placeholder for a conversation, built on three Cs described by Ron Jeffries.
Card
The Short Version
“As a business customer, I want to download my invoice as a PDF, so I can send it to my accountant.”
Who, what and why. Short enough for a card.
Conversation
The Detail
“Which invoices? Just the latest? What must the PDF show?”
The details come from talking, not from the card.
Confirmation
How We'll Know
Acceptance criteria agreed before building (Day 33).
The conversation, written down as tests.
Source: RonJeffries.com: Essential XP: Card, Conversation, Confirmation ↗
Card, conversation, confirmation.