Can 6.4 and 6.7 funds be utilized together on the same prototype development OTA? If so what authority? Would the OTA have a 6.4 CLIN and a 6.7 CLIN?
The DoD OT Guide dated Nov 2018 does not restrict the usage of any Budget Activity Codes. In fact, it says "To determine the appropriate funding type, the intent and stage of development of the effort should be considered and the Government team should consult with fiscal managers, agency legal counsel and comptrollers. Multiple funding types may be appropriate depending on the intent and stage of the effort." If both of those BACs will be used, then determine that they can be tied to the scope of the OT.
Since most RDT&E funding comes through Title IV of the annual defense appropriations act, check with Finance/Comptroller to see if there are any statutory restrictions in the applicable NDAA tied to your funding. If there are no statutory restrictions, then the DoD Financial Management Regulation (FMR) and any applicable agency policy will be the regulatory source to determine using this codes.
DOD Financial Management Regulation (Volume 2B, Chapter 5):
BAC 6.4 Advanced Component Development and Prototypes "Efforts necessary to evaluate integrated technologies, representative modes, or prototype systems in a high fidelity and realistic operating environment are funded in this budget activity. The ACD&P phase includes system specific efforts that help expedite technology transition from the laboratory to operational use. Emphasis is on proving component and subsystem maturity prior to integration in major and complex systems and may involve risk reduction initiatives. "
BAC 6.7 Operational System Development "This budget activity includes development efforts to upgrade systems that have been fielded or have received approval. for full rate production and anticipate production funding in the current or subsequent fiscal year. All items are major line item projects that appear as RDT&E Costs of Weapon System Elements in other programs. Program control is exercised by review of individual projects. Programs in this category involve systems that have received approval for Low Rate Initial Production (LRIP)."
Since DFARS will not apply to establishing the CLIN structure, think about the pros and cons of having one CLIN versus two separate ones. Also, given these BACs are for different efforts of RDT&E, that may drive the need for separate CLINs so funding and contractor payments can be easily tracked and managed.