What you are looking for is cascading validations.
There are a few articles on this.
Are you looking at make something like the following?
A drop down that has a set of options, and based on that selection a second drop down appears?
I have one in testing that looks like this.
If the person selects 1 on Test another box appears
I did this by using the visible option on the box for drop1
Visible = Expression
combo_2 == 1
combo_2 is my drop1 field.
Hope that helps