In a model you need multiple drop down menus, the choice in drop down menus is dependent upon each other. Like if ones chooses Asia (in 1st menu), then only Asian Countries be available as second choice (2nd menu); for Europe only European countries be prompted.
You
have some data where a particular field has many items occurring repeatedly and you need
a list of unique items only like from (A,B,A,C,D,E,F,F,E,C,B,A,D) you need
(A,B,C,D,E)