What's the difference?
They have two different purposes.
Document Access Codes provide extra security for signnature request links and document download links. The same code is used by all signers and may be sent to the signers by any means you choose or by an SMS sent by CrunchSign. They therefore do not necessarily provide useful evidence of a signers identity.
Signer PIN codes provide extra evidence of the signer's identity. Each PIN is unique and only known to the signer. They are always sent by CrunchSign via SMS.