The 45-day exemption-certificate rule
In short
Forty-five days from an auditor's request to produce a missing exemption certificate. A certificate that was available on the transaction date, fits the item and the purchaser's business, and is correct for the jurisdiction relieves the liability. The rule is in ADOR's audit guidance, not the statute.
In full
Where an audit finds a claimed exemption with no certificate behind it, ADOR's stated practice is that the auditor gives the taxpayer forty-five days from the request for substantiation to do one of two things: obtain a fully completed exemption certificate that was statutorily available on the date of the transaction, applicable to the item purchased, reasonable for the purchaser's type of business and correct for the jurisdiction the transaction is sourced to; or obtain other information establishing that the transaction was not subject to tax.
The relief is stated in mandatory terms. If the taxpayer obtains either, the auditor shall relieve the taxpayer of any liability for the transaction tax, unless the audit shows the taxpayer had knowledge of false information relating to the claimed exemption.
The certificate must have been available on the transaction date. A certificate that did not exist then cannot be produced now to cover it, so the cure is a documentation cure and not a reclassification.
A taxpayer who does not obtain a properly completed certificate carries the burden of proof to establish the required information by other means.
What makes the rule worth knowing is where it lives. A.R.S. § 42-5009 states the certificate framework and the fallback burden of proof, and states no period and no entitlement to relief on producing a certificate later. The forty-five days and the relief that follows appear in ADOR's audit guidance.
It is also not the protest window, which is a different forty-five days measured from a different event.
ADOR TPT Audit; A.R.S. § 42-5009(A), (B)