I'm not 100% sure, but I imagine it is for this use case.
Imagine you have a sales queue, but some of the calls in the sales queue is transferred from another queue.
When you filter the data on the Sales Queue, do you want to see just calls where Sales queue was the first queue, or include calls transferred from other queues?
Transfers can really make things difficult to understand specific scenarios/metrics, especially when you have many queues selected, that can all have transfers to each other. So selecting First Queue only just simplifies that.
Think of prioritization, we prioritize internal transfer calls, so when measuring ASA it makes more sense for us to only look at when it is the first queue, ie not internal transfer, as that may skew the results to be lower than what customers actually experience.
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Anton Vroon
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