Which Task field do you use to relate it to the Scenario it came from during the disaggregation exercise at the beginning of the iteration?
A: you do this by creating the Task using the command "Add Related Work Item" from the Team menu. The link to the original Scenario will appear in the Links tab of the Task work item.
Why the Scenario WI does not contain a field to capture the user acceptance test related to it?
A: This is more of a methodology design question. At the time of its design, the original MSF for Agile Software Development creators opted for not adding a UA test field to keep the Scenario work item lighter. So the answer is the MSF Agile 4.0 does not have the field but that does not prevent you to add one if you wish by customizing the template into your own.
Which Scenario/Task field can I use to set the priority? Rank??
A: Rank is the right answer for MSF Agile 4.0. For MSF CMMI 4.0, Priority field is the right answer.