How we create dynamic code list in extention BO

Hi

I am extend two field in xbo

field 1-Course Type(Target,Foundation)

field 2-Course Name(Target-JEE mains+Advanced,Medical UG & Foundation- Board+JEE,Board+Medical)

on the basis of Course Type i want to select Course Name.I am using BCO and BC set but it is not happening.

In Field 2 i am confused what data type i should select. And i do not want to create embedded component

Plz give proper solution.

Thanks

Sanjay