R12 - Operation Status In Debrief Material

Can someone clarify for me the difference between "In Service" and "Installed" when it comes to listing Materials and the Operation Status?  We have lots of stand alone parts which are installed at our customers locations, so when the parent instance defaults it usually needs to be removed, correct, as it is a stand alone part.  In turn, I assume the operation status is "installed".  Our Customer Service team is looking for clarification and the manual was not very clear on what the difference was between the two.

When the part or asset installed is without reference to a parent, in other