There is a view among philosophers that concrete particulars are wholes that are made up of metaphysically more fundamental constituents. From this there have been two main competing theories: substratum theory and bundle theory.
Concrete particulars is a whole made up of various properties associated with the particular together with an underlying subject or substratum that has an identity independent of the properties - a bar substrata. This view is a big odd because the properties that make up a concrete particular are the ones exemplifying the property and not the particular. So a red ball is not red. The property of red is exemplifying red, the ball itself isn’t. And this also applies to it being circular. Each attribute is its own subject.
One of the main critiques of substratum theory is the notion of bare substrata, which is a thing that has no characteristics and merely a tool for explaining numerical entities.
There is no underlying substrata. Ordinary particulars are constituted exclusively by the properties associated with them. They are just bundles or clusters. An objection to this by realists is the view that objects can change, but if an object is merely a bundle of attributes - this would mean a change is a completely new particular. Another objection is the notion of subject-predicate discourse. When we produce a sentence like this the ball is red we are referring to an object and not merely a bundle of attributes.
There is another theory that differs from that of the previous two that Aristotle came up with. Substratum and bundle theory have their problems and objections. Aristotle referred to things like plants, animals, and persons as fundamental entities that cannot be reduced to more basic entities. This particular view denies the constructivist approach of concrete particulars having underlying pieces - both of which substrata and bundles are.
Substance theory includes the use of kinds. These are universals, but ones that a concrete particular belongs. The concrete particular Socrates belongs to the kind human being. A big benefit of this is that there can be a near endless list of attributes that make up an individual human, but with the kind human being one doesn’t have to have an endless list of attributes - and still know one is a kind human being. It also deals with the numerical aspect of concrete particulars.