Check in revision option of Read from File

I was just wondering if anybody has used the revision opitionof "Read from File"? I am setting up PDM works for our very busyengineer and he was asking what exactly that option does when he ischecking in a file.

Anybody please advice.
I can't find any references to it in the help files.

Does is mean that it checks in the saved file rather than thechanged part you are working on in Solid works?
SolidworksWorkgroup Pdm