To co-sign a document means that a responsible person need to sign a document in addition to the user. It is a double signing used for important documents or to confirm that a user has participated to a course or has got a sertificate.
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If you should co-sign a document you will find this on the To Do Panel or My documents.
Select the document
Click on the V for co-signing
Or click on the X for rejecting
When you co-sign you can add a comment to the user you are co-signing for.
The comment is visible for the user only who find it from "My documents".