The importance flagging feature in BlueTie is so that the creator of an email can designate the importance level from low, medium and high. This is only set by the BlueTie user who creates the email, and can't be changed by the recipient of that email.
Priority flagging in Outlook is so that a user can flag an urgent email they have received as such, and can control it within their own mail client as they wish by increasing or decreasing their priority.
These flags are compatible so that they are displayed in both Outlook and BlueTie when set by the sender.