the IIO's have told me that you need to get AP if you have any urgent travel needs. i am not sure what they classify as urgent but i am sure they classify business travel 'good' enough reason to file for AP. so, if you have a company letter for travel, that should be good. but for personal reason, i have heard that many people would take their itinerary with them. if you have tickets that should be good. i have not heard about them asking for a proof of why you are traveling. meaning i have not seen any body taking their parent's medical record to prove they need to travel to visit their sick family etc.
that is if you are going to the local office for AP. when you apply AP through VSC, they really do not need any itinerary etc. but your company attorney would normally mention that you may travel on business.