I don't think this is documented anywhere, simply because, your definition of "user impact" is different than another person's definition of "user impact"
Here, I'll give you an example:
If you were to restart the CAD Entrerprise service to fix a stuck ECC layout/display issue, you may think that because the Agent doesn't get kicked out, and the calls stay active, that this has no "user impact". I on the other hand, rely heavily on CAD actions, and while the Enterprise service is down, no data comes into CAD for my call, and my macros don't work. Now, I cannot do my job, because I'm not really familiar from where that magical form comes from, that usually pops itself up automagically upon every call.
Ok, that was way more wordy than it needed to be, but I felt like typing... =)