In the Catholic rite, the bread _becomes_ the body of the Christ during Eucharist, a process known as _transubstantiation_ (meanwhile in protestant liturgy, it _contains_ the body of the Christ, a process known as _consubstantiation_). In any case, the bread is more than just bread, it's the symbol of the sacrifice made by the Christ to save Humanity and renew the alliance made with God. I think (I'm not a doctor of canon law by the way) that's what it means: the bread beyond its substance.
The Catholic theological tradition holds that the "form" remains bread, but the "substance" becomes God. It is explicitly not a symbol, it is God. Many of the laity don't understand this or, even if they do, do not think it a necessary article of faith.
This reminds me of a phrase I've heard many times, but I don't know the origin. "Protestants believe Eucharist is symbolic. Catholics believe the Eucharist is real. Orthodox Christians believe the Eucharist is real because it's symbolic."
The word "symbol" originally meant "where two things come together" as in, two rivers joining into one river. Many Protestants believe that the Eucharist is only a token or reminder of Christ. The Catholics have developed the idea that it materially changes. The Orthodox believe it changes into the Body of Christ, but not in a way you could physically measure.
I'm not a theologian and this is my poor layman's understanding.
It may have developed, but it appears to have developed quite early, if you read the writings of the 1st and 2nd century. It was reading Ignatius of Antioch (or maybe Polycarp) that made me realize how early the Catholic / Orthodox understanding was.
Catholics do not believe you can physically measure the change to the eucharist. The aristotelian understanding and explanation of transubstantiation is not a requirement of the catholic religion as emphasized by the unity of the eastern catholic churches who do hold on to the dogma that the bread and wine are indeed the body and blood of Christ but teach nothing as to how that takes place.
