What modification to a request is considered a 'response'?

What modification to a request is considered a 'response'?

Hello all,
 
I've recently enabled the response SLA measure within our organisation (previously only using resolution).  At what point is a request considered to be 'responded to'?  What needs to be changed in the request?
 
An example: A request logged by a user has been assigned to a technician, had it's priority set, and had a public note added, and it still breached the response SLA..
 
Many thanks!

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