Based on the screen functionality, we will write the unit 
test cases. Let's say, I have to test a GUI based screen, I 
will write the test cases like, when the screen is getting 
loaded what is the status of screen, What are the fields 
are enabled and disabled etc.,  Each field length comparing 
with the database. Acceptance of each field like AccountNo 
it should accept 6 letters and  it should allow only 
numeric etc., It should not allow alphanumeric and special 
characters etc.,