Let me break it down:
When you start, you’ll be in Tier 1, which means you can send messages to up to 1,000 unique users per day.
If your messages get good engagement (meaning people reply, don’t block you, etc.), Meta will automatically move you to Tier 2 — allowing 10,000 messages/day.
Then comes Tier 3, which unlocks 100,000 messages/day, and finally Tier 4 (unlimited messaging, based on your business needs).