AN excerpt from "The Necessary Signification" by P.H. Omlor:

There are three "things" which are involved in the very make-up of all the Sacraments. These are certain features or aspects or "things," which we prefer to designate as "cornerstones." The three cornerstones are defined and explained as follows.

(1) The "sacramentum tantum"- which is translated as "the sign only." The sacramentum tantum has a role that is solely an active one. It is that cornerstone which has only one function (which is why it is called the sign only), namely, to signify. But it signifies two different realities; that is, two distinct and separate "res's" -- as will be explained.

Furthermore this sacramentum tantum consists of two components, both of which are already familiar to us, namely, the matter of the Sacrament, and the form of words that determine the purpose of the sacramental matter. St. Thomas calls the form the "determining principle." These two components -- the matter and the form -- always act together in the process of signifying; they must be "morally united." In the Sacrament of the Holy Eucharist, for example, the sacramentum tantum is the bread and wine joined with the Words of Consecration, which are the form of the Sacrament.

(2) The "res et sacramentum", that is, "a reality and a sign." This cornerstone fulfills two roles, an active one and a passive one. First of all, it is a reality, a res, because it is one of the two separate and distinct realities that are signified by the sacramentum tantum; that is its passive role. But although it is a reality that is itself (by definition) something that is signified, it in turn signifies something else again, thus fulfilling its active role, namely, as a sign. Hence it is readily understood why this cornerstone is said to be both "a reality and a sign" -- it is signified and it also signifies. Unlike the sacramentum tantum which consists of both matter and words (the form), the res et sacramentum is a silent sign, for it signifies by its very presence, without any words being necessary. In the Sacrament of the Holy Eucharist the True Body of Christ is the res et sacramentum.

(3) Finally, the res tantum, which means "the reality only," is the third cornerstone. It does no signifying; its nature is solely that of a res; hence, res tantum. It is, so to speak, hit from two different directions. It is signified twice; first by the matter and the form of the sacramentum tantum; and also without words by the res et sacramentum.

Instead of res tantum it is often called by that other term with which we are already familiar, namely, the "Res Sacramenti," which is translated as "the Reality of the Sacrament." Notice the definite article, "the"; it is the reality of the Sacrament, not just a reality. Other ways of expressing this are "the effect" or the "grace proper" of the Sacrament. This third cornerstone, the effect of the
Sacrament of the Holy Eucharist, is the union of the Mystical Body. Or perhaps it is better said that the effect on the recipient is his more intense and firmer incorporation in the Mystical Body. This effect is sacramental grace.