Tenn. Code Ann. § 67-4-2010: Taxation of persons doing business outside state

(a) Any taxpayer having business activities that are taxable both inside and outside this state shall allocate or apportion its net earnings or losses as provided in this part. A taxpayer is considered taxable in another state only if the taxpayer is conducting activities in that state that, if conducted in Tennessee, would constitute doing business in Tennessee and would subject the taxpayer to either Tennessee's franchise tax or excise tax.

(b) Nonbusiness receipts shall not be included in the numerator or denominator of any apportionment formula.