Sec. 21.03.  INTERSTATE ALLOCATION.  (a)  If personal property that is taxable by a taxing unit is used continually outside this state, whether regularly or irregularly, the appraisal office shall allocate to this state the portion of the total market value of the property that fairly reflects its use in this state.
(b)  The comptroller shall adopt rules:
(1)  identifying the kinds of property subject to this section;  and
(2)  establishing formulas for calculating the proportion of total market value to be allocated to this state.
Acts 1979, 66th Leg., p. 2247, ch. 841, Sec. 1, eff. Jan. 1, 1982.  Amended by Acts 1991, 72nd Leg., 2nd C.S., ch. 6, Sec. 14, eff. Sept. 1, 1991.