An instance is a way to populate a schema with data. For each entity, it describes which are its possible values.

An instance is described providing some generators and some equations, like in the following example

instance MyInstance = literal : MySchema {
	generators
		a b c : Employee
		m s : Department
	equations
		first(a) = Al
		first(b) = Bob  last(b) = Bo
		first(c) = Carl
		name(m)  = Math name(s) = CS
		age(a) = age(c)
		manager(a) = a manager(b) = c manager(c) = c
		worksIn(a) = m worksIn(b) = s worksIn(c) = s
		secretary(s) = c secretary(m) = a
		secretary(worksIn(a)) = manager(a)
		worksIn(a) = worksIn(manager(a))
		age(a) = "1"
		age(b) = "5"

where each generator correspond to a "row" of its entity "table" and the equations define the attributes and the forreign keys of the entities, in such a way that the path_equations of the schema are satisfied.

An instance is a functor from the category presented by the schema to the category Set. This means that for every entity we need to associate a set of elements, which can be tought as the rows of the table. Moreover, for every foreign_key of the schema, which is nothing else than a morphism in our starting category, we need to associate a function in the Set category. Specifically, if we have a foreign key worksIn : Employee -> Department, we need to define a function from the set of Employees to the set of Department. In other terms, we need to say, for every employee, to which department he belongs to.