Burden of proof
In short
Who has to prove what. The taxpayer carries it for deductions and exemptions, with one contracting exception that runs against the Department.
In full
A taxpayer claiming a deduction establishes entitlement to it, and A.R.S. § 42-5009 gives two routes. Under subsection (A), the seller marks the invoice and obtains a completed certificate from the purchaser. Under subsection (B), a person who did not do that may still establish entitlement by presenting the facts necessary to support it, but the burden of proof is on that person.
In an audit the same rule appears as a practical one. Where the taxpayer does not obtain a properly completed exemption certificate, ADOR states that the taxpayer shall have the burden of proof to establish the required information.
The certificate is not the entitlement. It is the evidence, and a taxpayer who lost it has not lost the deduction — only the easy way of proving it.
One contracting provision reverses the direction. A.R.S. § 42-5075(S)(1)(c) places the burden on the Department where it asserts that a contract was artificially separated, so a contractor who wrote two genuine contracts is not required to prove a negative.
A certificate accepted in good faith also moves the burden between the parties rather than removing it. Under § 42-5009(O), a vendor who accepts in good faith is relieved of the burden of proving entitlement, and the purchaser may then be required to establish the accuracy of the claimed exemption.
A.R.S. § 42-5009(A), (B), (O); A.R.S. § 42-5075(S)(1)(c); ADOR TPT Audit