Notification - Acknowledge requester by e-mail when the request is updated - what does update mean?

Notification - Acknowledge requester by e-mail when the request is updated - what does update mean?

Our users interact with SDP mostly via email. I have had users complain that they do not want to see email notifications when they reply to request notifications. So I turned off that feature and turned on the feature "Acknowledge requester by e-mail when the request is updated".
 
My question is - what triggers this? What is the definition of "updated"? I made myself an owner of a request, and set someone else as technician. I then had another technician reply to a notification and I did not get an email notification of this "update".
 
I would appreciate a definition of what "update" triggers this notification rule.
 
Running SDP 8006.

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