Sales person and Responsible person

Why does TBM get the Sales person and Responsible person from the Vendor list and not from the user base? I can't quite grasp why it was built this way ?
If, for example, you want to assign a Sales person (who already has an account in the system) to a given Quote then you still have to create a Vendor, which is lost and confusing in my opinion, as a rule a Vendor is more like an Interpreter. Can I ask you to elaborate on your doubts ? perhaps I misunderstand these fields ?

Regards,
Patryk

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