I think he/she means a manual that tells one how to do repairs or service or modifications to the iMac. Like how to remove the hard drive or how to change the optical drive, etc. As opposed to a user guide that tells you how to use the Mac.
Except that one is opened from the back (A+B) and the other is opened from the front (C + intel) and you have to move the screen to get at the innards. Plus the latter two had a complete internal redesign.