Union, in general, is the joining together of two or more things, so that in some way or other they become one. The hypostatical union consists in joining together the divine and human natures of Christ, so as to constitute but one person. The union which holds between the sign and the thing signified in the sacraments is called a sacramental union; and it is of this that we must now speak. The Papists imagine that the signs which are used in the celebration of the Lord's Supper, are changed into the things signified. But a change is no union. It is necessary, also, that a sacramental union should correspond with all sacraments, or else it will not be sacramental, but will have reference merely to baptism, and the eucharist, and so be no longer general in its nature. Others suppose that there is a corporal conjunction, or union between the sign and the thing signified, as if they were one mass, and as if both existed at the same time in the Same place. But such a co-existence as this, and concealment of the one in the other is no sacramental union, for the reason that it does not agree with sacraments generally. A sacramental union, therefore, is not corporal, nor does it consist in the presence of the sign and the thing signified in the same place; much less in trap, or con-substantiation; but it is relative, and consists in these two things:

1. In a likeness or correspondence between the signs and the things signified thereby, concerning which Augustine says: "If the sacraments had not a certain resemblance or relation to the things of which they are sacraments, they would not be sacraments."
2. In the joint-exhibition and reception of the signs and things signified in their proper use, which cannot be done without faith, as we shall hereafter show. None but those who have faith receive from the minister the signs, and from Christ the things signified ; and when they thus receive both in their proper use, we have what is called the sacramental union.
   

This is proven, first, from the nature of a sacrament. The word sacrament is relative. The rites and ceremonies which God has instituted constitute the foundation or ground-work. The term includes Christ, and communion with him in all his benefits. The relation, is the order or connection which exists between the rites and the things which they signify. The correlatives are the signs and the things signified. From this, it is evident that the sacramental union is nothing else, than the relation which the sign has to that which is signified, from which we obtain this infallible rule: While this relation continues the sign and the thing signified remain united; but when it once ceases, they are no longer united; by which we are to understand, that as long as the order established by God between the sign and the thing signified remains, so long are the things exhibited and sealed with the signs; but when this divine appointment ceases, the signs do not exhibit or seal anything unto us. The second proof which we advance in support of the sacramental union as just explained, is that which arises out of the analogy and correspondence of sacraments. It must be a union in harmony with all sacraments. Let us, therefore, inquire, what was the union between Christ and the sacraments of old, and we shall then see what is the nature of the union which holds in the sacraments of the New Testament; for there must be a correspondence in this respect, or else the sacraments of old were no sacraments, or the union was not sacramental, not being such as corresponds with all sacraments. The union now which belonged to the sacraments of old could only be a respective or relative union. Hence, such must now also be the nature of that union which is sacramental.

--Zacharias Ursinus, Commentary on the Heidelberg Catechsim, p. 348-349.