Article II, Section 3 of the Constitution states the president "shall commission all the officers of the United States," meaning the president must sign official documents authorizing federal officials to assume their duties. This seemingly ceremonial requirement became the center of constitutional conflict in the landmark 1803 case Marbury v. Madison. President John Adams signed judicial commissions for William Marbury and others in his final hours in office, but incoming President Thomas Jefferson ordered Secretary of State James Madison not to deliver them.
The commissioning power has three phases: first, the president nominates an official; second, the Senate confirms (for positions requiring confirmation); third, the president signs and delivers a commission. The Supreme Court in Marbury held that signing the commission is the "final act of appointment" for principal officers. While Article II says the president "shall" commission officers, the power operates at presidential discretion, meaning the president chooses when to commission but cannot delegate that authority to anyone else. This gives presidents control over the timing of appointments and leverage in negotiations with appointees. Lower-level officers may not require formal commissions, but Cabinet secretaries, ambassadors, federal judges, and military officers all receive signed commissions from the president.