Second Vatican Council: “Although the individual bishops do not enjoy the prerogative of infallibility, they nevertheless proclaim Christ’s doctrine infallibly whenever, even though dispersed through the world, but still maintaining the bond of communion among themselves and with the successor of Peter, and authentically teaching matters of faith and morals, they are in agreement on one position as definitively to be held.” (Lumen Gentium 25).
This particular type of exercise of the Sacred Magisterium is often called ‘ordinary and universal’ because it takes place in the course of the daily teaching and witness of the Bishops dispersed throughout the world yet united with the Pope, and not in the course of a particular gathering or a particular document. First, many Bishops, individually or in local groups, in various ways, at various times, exercise the non-infallible Ordinary Magisterium on a particular point of doctrine. Eventually, this non-infallible teaching becomes taught universally, both by Bishops throughout the world, and by the Pope as their head. When such a teaching, which initially was taught under the non-infallible Ordinary Magisterium, is finally taught definitively by the Bishops dispersed through the world and by the Roman Pontiff as the head of the Bishops, then this ordinary teaching becomes a universal teaching, and therefore infallible. For the whole body of Bishops led by the Pope cannot err when they are in agreement on one position of faith and morals definitively to be held by all the faithful.
The Universal Magisterium is exercised when the Bishops dispersed through the world, and the Pope also, teach one and the same doctrine, from the Deposit of Faith, as a required belief for all the faithful. Such an exercise occurs over time within the daily preaching and witness of the Bishops and the Pope. It may find various written expressions in various times and forms, but those particular expressions are not themselves an infallible exercise of the Sacred Magisterium. The universality of such a definitive teaching is what moves the teaching from falling initially under the non-infallible Ordinary Magisterium to fall finally under the infallible Sacred Magisterium.
The Universal Magisterium cannot be exercised apart from the Pope because the Pope is the head of the body of Bishops. If no Pope has ever taught or witnessed to a particular teaching, then it is not a teaching of the Universal Magisterium, nor does it fall under any exercise of the Sacred Magisterium.