MongoDB Kafka Connector: Message key and partition strategy

We have these settings in our MongoDB Kafka Source connector:
output.schema.key={ “type”: “record”, “name”: “keySchema”, “fields” : [ { “name”: “documentKey”, “type”: [“string”, “null”] } ] }

What exactly is the message key? Does the producer use the default partitioner?

The consumer gets a string like Struct{documentKey={"_id": “”}}, but if we produce a message with the same string as key, it goes to a different partition.