Below is a quick breakdown of signature_request_signed and signature_request_all_signed events:
The signature_request_signed event indicates a signer has signed the document. This event will be POSTed to your callback URL each time a signer has completed their assigned fields in a document, e.g. if there are 10 signers on a single signature request, this event will be POSTed 10 times (once for each signer).
The signature_request_all_signed event indicates that all signers have signed the document. Once this event is POSTed to your callback URL, the document will be ready for download.
To prevent downloading incomplete signature requests, or encountering errors when attempting to download documents, we recommended waiting for the signature_request_all_signed event.