sorry if this sound stupid but I'm new to this business.
after reading this forum this is what i understand:

agents: you put your headshot + resume + cover letter

casting directors: just headshot + resume
(no need for a cover letter)

am i correct?

No, you're not correct. You should always include a cover letter. Just keep it short and sweet. Maybe you can reference the project the CD is working on.

And to be honest, if you're this new to the business, you probably shouldn't be submitting to agents yet. It doesn't sound like you're ready for professional representation.