Defense Primer: DOD Transfer and Reprogramming Authorities [Updated February 14, 2020] [open pdf - 509KB]
From the Document: "In provisions of the 'U.S. Code' and on an annual basis through 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."
CRS In Focus, IF11243
Congressional Research Service: https://crsreports.congress.gov/