Pulsar Functions appears to be a stream processing engine similar to Flink or Kafka Streams. In other words, it is more like a runtime for streaming apps.

The relation Pulsar ~ Pulsar Functions appears similar to Kafka ~ Kafka Streams. One is a message broker, the other a streaming engine.

Exactly-once (or effectively-once) in Pulsar Functions is depended on assigning unique sequence ids on the producer side. When consuming from Pulsar topic those can be taken from the consumed message, but AFAIK Pulsar doesn't assign those unique ids on its own, so it needs to be done externally.