Here's an explanation from the Catholic Encyclopedia:
What teaching is infallible?
As regards matter, only doctrines of
faith and
morals, and facts so intimately connected with these as to require infallible determination, fall under the scope of infallible
ecclesiastical teaching...
As to the organ of authority by which such doctrines or facts are determined, three possible organs exist. One of these, the
magisterium ordinarium, is liable to be somewhat indefinite in its pronouncements and, as a consequence, practically ineffective as an organ. The other two, however, are adequately efficient organs, and when they definitively decide any question of
faith or
morals that may arise, no believer who pays due attention to
Christ's promises can consistently refuse to assent with absolute and irrevocable
certainty to their teaching.
But before being bound to give such an assent, the believer has a
right to be certain that the teaching in question is definitive (since only definitive teaching is infallible); and the means by which the definitive intention, whether of a council or of the
pope, may be recognized have been stated above. It need only be added here that not everything in a
conciliar or
papal pronouncement, in which some
doctrine is defined, is to be treated as definitive and infallible. For example, in the lengthy
Bull of
Pius IX defining the Immaculate Conception the strictly definitive and infallible portion is comprised in a sentence or two; and the same is
true in many cases in regard to
conciliar decisions. The merely argumentative and justificatory statements embodied in definitive judgments, however
true and authoritative they may be, are not covered by the guarantee of infallibility which attaches to the strictly definitive sentences — unless, indeed, their infallibility has been previously or subsequently established by an independent decision.
From
Fundamentals of Catholic dogma by Ludwig Ott quoted from Wikipedia, "
Dogma in the Catholic Church":
Theological certainty Description
1.
de fide Divine revelations with the highest degree of certainty, considered Divine revelation (and infallibly asserted)
2.
fides ecclesiastica Church teachings, which have been definitively decided on by the Magisterium in an infallible manner
3.
sententia fidei proxima Church teachings, which are generally accepted as divine revelation but not defined as such by the magisterium
4.
sententia certa Church teachings which the Magisterium clearly decided for, albeit without claiming infallibility
5.
sententia communis Teachings which are popular but within the (filtered) range of theological research
6.
sententia probabilis Teachings with low degree of certainty
7.
sententia bene fundata A well-reasoned teaching which does, however, not arise to being called probable
8.
opinio tolerata Opinions tolerated, but discouraged, within the Catholic Church