If two appropriations are equally applicable, the agency may choose either one. Be mindful that the Financial Management Regulation (FMR) advises in favor of the RDT&E appropriation if doubt exists whether it or another appropriation is properly chargeable (FMR Vol 2a 010213 B). From GAO case citations, once the election of an appropriation is made, the agency must continue to use that appropriation until the end of the current fiscal year. That election of funds is binding even if the agency runs out of funds. If the agency wants to change appropriations for the next fiscal year, it must notify Congress of its intent.