Employees are real persons who can be characterized with many
properties; very few are: name, size, date of birth, shape,
social number, room number, hair colour, hobbies.
Not all of these properties are necessary to solve the administration
problem. Only some of them are problem specific. Consequently
you create a model of an employee for the problem. This model
only implies properties which are needed to fulfill the requirements
of the administration, for instance name, date of birth and
social number.
These properties are called the data of the (employee) model.
Now you have described real persons with help of an abstract
employee.