What is the difference between "Place Signature" and "Sign Document" when working with PDF-XChange Viewer?

 When I go to the menu item Document --> Signing I see two items; "Sign Document" and "Place Signature" but they appear to do the same thing.  What is the difference between them?

The reason both "Place Signature" and "Sign Document" appear to perform the same thing is because PDF-XChange Viewer can only place a signed signature field, it cannot place an unsigned signature field. It can however sign an unsigned signature field that has been put in place using other methods.
 

So "Place Signature" will create and place a Digital signature/Certificate on the document. "Sign Document" will usually work the same way as "Place Signature" unless there exists already in the document an unsigned signature field. If there is only one of these in the document then "Sign Document" will go straight to the "Sign Document" dialogue and add the signature to the existing signature field.  If there is more than one unsigned signature field then "Sign Document" will pop up a message to the effect that there is more than one to choose from and to please select the one you want to sign, then the sign document dialogue will again open.

The neat thing about signing a PDF this way is that it 'fixes' the state of the document at the time of signing.  This is why it forces a save of the document. What we have now is a signature on a document in a specific state. If that document is marked up and signed again it will be saved in the new state. It is theoretically possible, though currently unsupported in PDF-XChange Viewer, to 'retrieve' a document with the state it was in when a given signature was applied.

So a scenario I could envisage would be to create a document that requires two (or more) users to approve and potentially modify and then "sign their changes". It would be technically possible to see exactly what a given user signed off on by looking at the state the document was in when it was signed.

To see this in action look at the 3 sample files in the zip file from the link below.  One is already signed, one with a single unsigned signature field and another with 2 unsigned signature fields.

http://www.tracker-software.com/forum3/download/file.php?id=2438