(1) For the purposes of these rules, the following terms have the meanings assigned unless the context dictates otherwise:

(a) "Principal business" means a business which, in relation to the transportation which is purported to be incidental, is clearly the predominant undertaking or enterprise. It must have a clear economic purpose or objective, identifiable with certainty, in terms of supplying goods or services.

(b) "In the furtherance of" means directly benefiting the principal business by transporting materials, goods, or other property, personnel, customers, clients, or other passengers, when such transportation assists the principal business in achieving its economic purpose or objective. It does not include a benefit to the principal business through transportation merely generating additional profit or like thing.

(c) "In the scope of" means directly within the bounds of the economic purpose or objective of the business. It does not include a transportation activity that is merely a tangent or appendage.

(d) "Subordinate to" means lesser than, minor in comparison to, dependent on, existing because of, and controlled by. It can include transportation important to, even essential to, the principal business. It does not include transportation which is a significant enterprise itself.

(2) A determination on whether these definitions are met or not is a question of law,