From the Document: "In the 'United States Code' and annual authorization and appropriations acts, Congress provides the Department of Defense (DOD) limited authority to obligate funds for purposes other than originally approved. These authorities allow the department to 'transfer or reprogram' funds. A transfer involves shifting funds from one appropriations account to another, while a reprogramming involves shifting funds within the same account. DOD uses the term 'reprogramming action' to describe both types of transactions. This primer focuses on authorities provided in the fiscal year ending September 30, 2018."
CRS In Focus, IF11243
Congressional Research Service: https://crsreports.congress.gov/