Kinds of authoritative Magisterial acts*
The Church's Magisterium teaches authoritatively by:
the Supreme Pontiff ex cathedra (literally 'from the chair', i.e. of St. Peter),
the College of Bishops in council, in solemn form, and
the ordinary Magisterium.
There is a difference between a doctrine being “definitively proposed” and one being taught by a “definitive act.”
“Definitive act” (or “act that is defining”) means definitively proposed by a single act in “solemn form”—either ex cathedra or by solemn definition of an ecumenical council, worded as, for example, “by the authority of Jesus Christ… and Our own.”
When there is no distinct “definitive act” a teaching may still be "definitively proposed" by a collective 'act' across space and time—even over many centuries and by various bishops throughout the world.
The Pope, without having recourse to an ex cathedra definition, may formally reaffirm or confirm that a doctrine has been definitively proposed because of the ordinary and universal teaching of the bishops, as in the Apostolic Letter Ordinatio Sacerdotali. The wording is: “based on Scripture, transmitted by the Church’s Tradition, and taught by the ordinary and universal Magisterium.” Effectively, the pope is here implying that no solemn single definitive act was necessary for the teaching that priestly ordination is reserved to men alone, because that doctrine had already been definitively proposed by the collective act of the ordinary and universal Magisterium over time. (Of course, nothing would prevent a Pope from a solemn definition in the future.)
Categories of Magisterial Teachings and the Responses they Evoke
There are three different categories of what is taught by the Magisterium, and we owe a different kind of assent to each.
I. De fide credenda (believed by Faith) first-category dogma.
These are truths recognized to be divinely revealed in Scripture or Tradition. They are definitively held based on Faith in the authority of the Word of God.
Examples of these are doctrines on the Creed, Sacraments, True Presence, True Church, Pope, Original sin, Immortality of Soul, Divinely inspired Scripture, Immorality of Murder.
They are Authoritative, Definitive, and Revealed.
They require assent of theological faith.
They incur censure of heresy for obstinate denial or doubt.
II. De fide tenenda (held by Faith) second-category doctrine.
These are truths regarding faith and morals are recognized as following from revealed truth, and necessary for faithfully keeping and expounding the deposit of faith.
They are definitively held based on Faith in the Holy Spirit as cause of the infallibility of the Magisterium.
Examples of these include Historically - Legitimacy of Papal election, council, canonizations; and Logically - Male ordination, Immorality of euthanasia, fornication, contraception.
They are Authoritative, Definitive, but not necessarily Revealed.
They require firm and definitive assent.
They incur censure of heresy for obstinate denial or doubt.
III. Obsequium religiosum (religious submission of will and intellect) third-category teachings.
These are propositions taught as conforming to the Faith, or when the contrary is dangerous to the Faith.
They are held out of reverence. (Not by assent of theological Faith, and not by the assent of human faith.)
Examples are not given but may be recognized "by the nature of the documents, by the frequent repetition of the same doctrine, or by the tenor of the verbal expression." They include contingent moral matters that change in the particular application according to place and time [like whether capital punishment should be employed?]
These are Authoritative, but not necessarily Definitive nor Revealed.
They require adherence according to the mind and the will manifested.
It is rash or dangerous (though not always erroneous) to hold the contrary. Tuto doceri non potest (they cannot safely be taught.)
Their rejection causes schism.
Infallible (guaranteed without error) - and therefore irreformable (unchangeable) -applies only to the first two categories of propositions taught. The first two categories are truths that simply are being recognized as already revealed or as following from what is revealed.
They do not necessarily require a solemn definition, because ordinary Magisterium is sufficient for infallibility, and irreformability.
In any of the three acts when a doctrine is “definitively proposed,” the proper response from the faithful will always be at least that it is “definitively held” by Faith, de fide tenenda. Third-category teachings are not (and sometimes cannot be) definitively proposed, and thus require a different kind of assent, or rather submission.
Can a teaching change categories, and therefore require a change of response from the faithful?
At any time a teaching in one category may, by a solemn definition, be upgraded to a higher category. A teaching that is recognized to follow from a revealed doctrine may later be recognized as itself revealed. Or a teaching that is promoted as conforming to the Faith may later be recognized as following from the Faith. One kind of third category teaching that can never require anything more than adherence out of reverence, and therefore can never be upgraded, is a proposition about contingent moral particulars.
Once a teaching is definitively proposed it is irreformable because it is infallible, and thus, never is a teaching downgraded to a lower category.