"Official assignment" refers to a task or duty that has been assigned by a person or an organization with authority or power. It implies that the task is formal, authorized, and recognized as part of one's responsibilities. Full definition
He worked as a freelance reporter, on official assignments, and illustrating record sleeves, but his real passion was studio work. (creativeboom.com)
For security reason, this is how the big man of his status goes out for official assignment. (justarsenal.com)
«The barricade was obviously a deliberate attempt to assassinate the Chief of Army Staff and members of his entourage while on a legitimate official assignment as Special Guest of Honour at the passing out parade which has earlier been widely publicized. (securitykingng.org)