Dataflow "mark as a key" VSwithout key column

Hi - 

According to official document "Using a key column indicates to the dataflow to upsert records into the destination table, while not selecting a key indicates to the dataflow to create new records in the destination table." I know dataflow follows upsert rule for the output with key column (mark as a key).

But I cannot find instructions for the output without key column. If output has updated value for a specific record which is existing in the destination like lakehouse or warehouse. How does it work when inserting?

Thanks in advance!