What is writeback and merge field in DocuSign Salesforce app?

Hi,

Merge Fields-
If information needs to be pulled from Salesforce and put into a DocuSign Envelope, Merge fields can be used to pull from Salesforce fields directly. Writeback can be used to push information back to Salesforce fields.

DocuSign Merge Field options:

1.Allow Writeback to Salesforce: Only available when Relate to Salesforce is selected. Select this check box to automatically update the Salesforce data when the data is changed by a signer.
2.Allow sender to edit custom fields: Only available when Relate to Salesforce is selected. Select this check box to let senders modify the value of the custom tag when the tag is added to a document during the sending process.

Thanks.