As part of the current Box Sign - Salesforce integration, signed documents and the signing log are automatically stored in the Box folder associated with the Salesforce record from which the document was sent out from. This enabled customers to ensure there was a single source of truth for all their content - including signed documents, and streamlined post-signature steps (classifying, retaining, securing, archiving).
On April 6th, we released the ability to pick a document in Box to send out for signature as well in the Salesforce Managed Package v3.76.
To use this enhancement, download and install v3.76 of the Box for Salesforce Managed Package from the Salesforce AppExchange.