As a general rule, first class items represent things rather than relationships.
For example, the database representations of a human and of a company are each first class items.
However, the fact that the person is an employee of that company is not a first class item.
Likewise, data about that relationship, e.g. information about the salary the company pays to its employee, is not a first class item.
In a table representing first class items, one column of the table will typically contain a different integer assigned to each row (effectively, to each item) as a unique identifier: that is to say, unique for objects of this type; objects of different types, represented in different tables, can coincidentally have the same identifier, but the coincidence is meaningless.