What is the use of constant of field in model of ormlite please see the detail quetion

i gone through on of tutorial of ormlite explained @ here in this tutorial in file StudentDetails.java there is field

public static final String TEACHER_ID_FIELD = "teacher_id";

but this field is not used in any where in that class. not only this example our project is also have to many such fields.

what is the propose of this and how they used?

thanx in advance