It is enabled if an external object has been selected and if you have the security profile EXTERNAL OBJECT ADMINISTRATOR with Change rights or if security rights have already been granted and a record for this exists on Doc_Obj_Security table with Check-Out rights. Clicking the Check-Out Button will open up a separate form on which the check-out details need to be recorded.
Notes - This field can be used to capture the Notes. The maximum number of characters that may be entered is 255.
New Revision - This field is used to capture the new revision. The maximum number of characters that may be entered is 50 (mandatory).