How to specify the Key for Bulk API Job 'delete' and 'update'?

Hi all,

I'm trying to use Job of Bulk APIs.
I found that, parameter 'externalIdFieldName' is required for 'upsert' to specify the Key field.
But I think that, 'delete' and 'update' also need a Key field.
How could I specify it when using Bulk APIs to create a Job?

Thanks