One of the big difficulties people often have when trying to navigate medieval philosophy is the notion of 'real relations' versus 'rational relations'. Part of this is because they are purely technical terms; the 'real' and the 'rational' don't at all mean what they would in English. Thus people think that 'real relations' are cases where you 'really' have a relation, as opposed to 'rational rational', where it is apparently all in the mind; but this is just a confusion. Here's a very brief guide to what a real relation is.
A few preliminary notes. 'Relation' here is also a technical term. We tend to think of relations as between things; these are what would have originally been thought as co-relations. 'Relation' here is something that applies directly to one term of a co-relation, not the co-relatedness itself. Thus 'being a father' and 'being a son' are distinct kinds of being-related-to, and each would be considered a relation in the original sense. Thus a relation is something you predicate of a subject, namely, that this subject is 'toward' another somehow, not something that you take to hold between two distinct subjects. Likewise, 'real' here is connected with res, which means what something substantively is, the 'something' that it itself is. The contrast here is with 'rational', from ratio, which in this context means something like the nature of the thing as it is understood or conceived. A real relation is a kind of relatedness qua res to something; a rational relation is a kind of relatedness qua ratio to something.
Keeping this in mind, a real relation has five aspects, all of which are necessary for a relation to be a real relation. A is related qua res to B when:
(1) A is related to B.
(2) This relatedness is due to some 'foundation' or actual aspect of A that makes the relatedness possible.
(3) A and B are not the same.
(4) The relatedness to B is 'in' A itself.
(5) The 'foundation' requires the relatedness to B.
The best way to think through them is to see what they exclude. Obviously, if (1) is not the case, there is no relation at all. If any of the others are violated, you have a rational relation.
An example of (2) being violated is that of an extrinsic relation. If A is to the left of B, this is not due to some actual aspect of A itself, but simply to how the two things happen to be situated relative to each other. In modern philosophy, a change in this kind of relation is often called a 'Cambridge change'.
An example of (3) being violated is self-identity. A's relations to itself are rational relations because when I say A is A, I'm not actually predicating another A of A, I'm just duplicating A mentally and thinking of that in terms of a relation.
An example of (4) being violated is that of a purely mental relation, such as how I associate two different things, which depends not on the things themselves but on my outside perspective on them.
The most famous example of (5) being violated is the relation between God and creatures. Creatures have a real relation to God, because they are related to God, who is different from them, by being because they are creatures and could not possibly be unrelated to the Creator. However, while God as Creator is related to creatures, God does not have a real relation to creatures because what makes God a Creator, His power and will, does not require that creatures exist at all. The relation of God to creatures is an actual one -- He is related to these creatures as Creator, this is due to His will, which is 'in' Him, so that they are not the same as Him -- but it is not a 'real relation' because the divine will is free to create or not to create. From this we see that many cases of making something are also not going to be 'real relations'.
