default, required, optional

[Bryan Duxbury]

Can someone help me understand the difference between required,  
default, and optional field modifiers? Required seems to make sense.  
Optional seems to make sense. However, the fact that there's a third  
state is quite ambiguous.

It seems like the field modifiers should be binary - required or  
optional. Leaving the modifier off should just be a shorthand for  
optional.

-Bryan