Work order
In short
The instrument that defines actual work under a master agreement. Under a Job Order Contracting arrangement each one is classified on its own.
In full
A work order is the document that specifies what will actually be built under an arrangement that did not specify it. The master agreement fixes duration, rates and a general scope; the work order fixes the work.
Under a Job Order Contracting arrangement, ADOR treats each work order as the unit of classification. Each is assessed separately as MRRA or Modification, each order involving alteration is tested against the alteration threshold on its own, and a mixed order is taxable in full unless its modification content is de minimis — the 15% test.
That is consistent with how contracts are treated generally: A.R.S. § 42-5075(P)(2) makes each contract independent of any other, and ADOR states that the taxability of a contract and of subsequent change orders is determined separately.
A work order is not a change order, and the two are decided differently. A change order modifies an existing contract, and one that directly relates to the original scope of work is treated the same as that contract. A work order under a Job Order Contracting arrangement is assessed on its own terms.
The vocabulary is not settled across sources. A.R.S. § 34-101(17) calls these instruments job orders; ADOR's Contracting FAQs call them work orders; both describe orders issued during the life of a master agreement to specify construction.
ADOR Contracting FAQs; A.R.S. § 34-101(17); A.R.S. § 42-5075(P)(2)