The reference is the method of procurement, not the amount. Sec.10 of law and its IRR mandates that all procurement shall be done through competitive bidding. There is no qualification as to amount. Even 10k worth of catering service should be undertaken through public bidding as a rule. In such case, Contract Agreement is mandatory.
However if, subject to prior approval of the HoPE and whenever justified under Rule XVI, alternative method is used such as Small Value Procurement, Contract Agreement form is no longer mandatory.
In both instances Purchase Order, which is a form prescribed by COA, is being required.