A document signer is a user on your account, set on a checklist, who is responsible for signing and/or approving forms for the employee to which the checklist is assigned.
 
If the form has two sides, an employee and a manager side, the person set as the signer will be required to fill out the manager fields of that form. When a form has only employee fields, the person set as the signer will need to view and approve the form after the employee completes it. Only after the manager approves the form will it be marked as complete and added to the employee's Documents page.

You will be asked to set a document signer when assigning an employee a checklist. On the Onboarding page, select Assign Tasks and then select the checklist(s) to assign and the associated document signer under the Signs/Approves Forms drop-down.

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User-added image

You may switch document signers at any time by following these steps as well — all remaining incomplete steps on the checklist will be assigned to the new signer.