I am trying to understand as to when this
ItemAdding event fires.
Does it fire when user has clicked new document button to upload a document library?
Does it fire when user uploads document library, then a form is shown where he enters Title etc., so before he presses Save button, this event is fired?
My aim of asking this question is to do some form validation after user presses Save button but before item is added. If there is some error in validation then I want to display error message on screen.