Volume 3 (of 5) of the Exegetical Commentary on the  Code of Canon Law (PDF pp. 3269-70) elucidates the "authentic magisterium" that appeared, from Lumen Gentium §25, in
1983 Code Can. 752
Although not an assent of faith, a religious submission of the intellect and will (religiosum tamen intellectus et voluntatis obsequium) must be given to a doctrine which the Supreme Pontiff or the college of bishops declares concerning faith or morals when they exercise the authentic magisterium (magisterium authenticum), even if they do not intend to proclaim it by definitive act*; therefore, the Christian faithful are to take care to avoid those things which do not agree with it.
*cf. Pius XII, Humani Generis §20: "Nor must it be thought that what is expounded in Encyclical Letters does not of itself demand consent, since in writing such Letters the Popes do not exercise the supreme power of their Teaching Authority. For these matters are taught with the ordinary teaching authority, of which it is true to say: 'He who heareth you, heareth me' [Lk. 10:16]; and generally what is expounded and inculcated in Encyclical Letters already for other reasons appertains to Catholic doctrine. But if the Supreme Pontiffs in their official documents purposely pass judgment on a matter up to that time under dispute, it is obvious that that matter, according to the mind and will of the Pontiffs, cannot be any longer considered a question open to discussion among theologians."
1. Classification of acts of magisterium with formally negative wording
The first comment that would seem to be necessary for a correct understanding of this canon is that both the intrinsic disposition of persons receiving acts of magisterium as well as the acts of magisterium themselves mentioned in c. 752 are described in negative terms. This approach in phrasing the law (which is necessary in this case, as we shall see) does not imply or suggest an attitude of inattention or unawareness with re spect to either divine assistance with the acts of magisterium mentioned in this canon or religious submission with which these acts are to be received.
Canon 752, like cc. 749-750, refers to the authentic universal magisterium of the Supreme Pontiff or College of Bishops. The difference is that c. 752 deals with magisterial acts the subject matter of which does not attempt to define a doctrine by means of a decisive infallible act. Magisterial acts of the Roman Pontiff that are made with no indication of being definitions of dogma are also called non ex cathedra magisterium (LG 25) or non-infallible pontifical magisterium. Along this same line of distinction, these acts of magisterium, as opposed to the acts of magisterium described in cc. 7 49-750, are also known as simply authentic magisterium, or magisterium that does not have the highest authority.
The need to resort to classifications of these magisterial acts that are technically negative (non-infallible, non-definitive, non ex cathedra) is necessitated by the fact that there are no specific bodies or kinds of action that can be exclusively described as those of the universal magisterium, as discussed here, to differentiate them from statements of dogma that constitute infallible magisterium. In fact, the same bodies (the Roman Pontiff and the College of Bishops) impart both infallible magisterium and non-definitive magisterium. Further, no distinction can be drawn between these two types of magisterium by stating that a solemn act gives rise to an infallible magisterium, since many documents written by ecumenical councils do not claim to define doctrine dogmatically. On the contrary, an ordinary and universal magisterium exercised by the bishops, "dispersed throughout the world but maintaining the bond of union among themselves and with the successor of Peter" (c. 749 §2) may be infallible (c. 750).
Further, the efficacy of a magisterial act alone to formulate a definitive, infallible teaching must not be confused with the fact that the same magisterial act, even though it may not expressly state that it is definitive dogma (such as an encyclical of the Roman Pontiff), may contain infallible doctrines originating from earlier definitive acts whose efficacy is not diminished because magisterial acts of lower authority recalls doctrines defined earlier.