In a previous post, I said that we should discuss the various aspects of things in order. I have already discussed properties and accidents (true of a thing in part and in common with other things). Tropes are those things that are not in common with other things and are true of a thing only in part. The other aspects of things are primary substances (not in common and not in part) and secondary substances (in common but not in part). Tropes must be distinguished from properties and accidents on the one hand and primary substances on the other hand.
Tropes are distinguishable from properties and accidents because they are not in common with any other thing. Consider a cup. The redness of the cup is accidental to the cup. However, it is possible for another cup to be red as well. If it were not, then the redness would not be an accident. It would be a trope instead. The cup is not merely red, however. Each thing in the world that is red is red in a different way. The unique way that the cup is red is the red trope of the cup. So for every single accident that a thing has, it also has a trope. The accident is universal because there could be more than one. The trope is particular because there can only be one of them. Properties also have tropes. Consider a cup that is formed to hold water. This is a property of the cup. However, not all cups are formed to hold water in the same way. Cups are made for different purposes. They are also made differently. These differences mean that they hold water slightly differently. So there is also a trope for this property. Therefore, every property and accident also has a trope that is the particular expression of the universal trait.
Tropes are distinguishable from primary substances because they are true of a thing only in part. Although every trait also has a trope, traits and tropes are both true of a thing only in part. This means that we are speaking of some aspect of a thing rather than another aspect of that thing. For instance, a tree’s being living does not completely describe the tree. People and dogs are also alive, and so are flowers and bugs. So when are speaking of a property, we are speaking of only one aspect of a thing. This is also true of accidents. Accidents are an even more obvious case of this. Redness is true of cups, but also of flowers, shirts and blood. So it cannot describe a thing except in regards to some aspect of it. Tropes are particular expressions of properties and accidents. Therefore, tropes refer to particular expressions of aspects of a thing. Primary substances, on the other hand, identify the thing itself as itself. They do not describe or refer to any particular aspect, but to the thing as a whole.
As far as what tropes are, this is enough to distinguish tropes from traits and primary substances.