to a State or Locality, regardless of whether such charge is denominated as a tax, a fee, or any other type of exaction. The term tax does not include any charge imposed by a State or Locality with respect to a concession agreement at a federally assisted airport (provided the agreement does not violate the revenue diversion provisions of section 47107(b), or the registration, licensing, or inspection of motor vehicles), if the charge is imposed generally with respect to motor vehicles, without regard to whether such vehicles are used in the business of renting motor vehicles within the State or Locality. (13) Tax base The term tax base means the receipts, income, value, weight, or other measure of a tax to which the rate is applied. The tax base of a tax imposed on a per unit basis is the unit. (14) Generally applicable tax The term generally applicable tax may be determined by— (A) the gross receipts of rentals of other tangible personal property or other commercial and industrial taxpayers within the State or Locality to which the tax is imposed by the State or Locality compared to the gross receipts of rentals of other tangible personal property or other commercial and industrial taxpayers within the State or Locality