A.R.S. § 42-5123

Nonaffiliated Indians and non-Indian vendors shall maintain sufficient documentation of their taxable and nontaxable gross proceeds and gross income, under any method of recording that properly reflects such taxable and nontaxable gross proceeds and gross income, to enable the department to determine which transactions are subject to transaction privilege tax. Any gross proceeds and gross income that cannot be identified as nontaxable is considered taxable.