One question about "New Producer Configs"

Dear sir or madam,

There is one question when I'm using Kafka:
From the "documentation:New Producer Configs" ,I get that acks=n means the number of acknowledgments the producer requires the leader to have received before considering a request complete. But why can the producer send messages successfully when I use the Producer API and the acks is set to 2 (the number of partitions is 3 but there is only one partition can work)?

Thanks a lot and look forward to hearing from you soon.

Best Regards,

Genlong Wang