Wasn't sure how to title this, but I've scoured the internet without any success finding exactly what I need to accomplish here. This is what I'm trying to do:

In a data tab, I have a column with a list of various plan names. The adjacent column classifies the plans into either complete (C), start (S), or finish (F). On the user interface tab, I want to require the user to select a plan in a pulldown list and it can be any plan in the list. If that plan is classifed as "complete" or "finish", the secondary pulldown list next to it is blank/greyed out/not applicable. But if they select a start plan in the first pulldown list, the secondary pulldown list should only display a list of appropriate finish plans.

Originally I only had a single drop down list and I've been using an OFFSET function in data validation to dynamically provide the list of plans. I need to maintain this method (not a pre-defined range) such that I can add or remove plans in the data tab at any given time without affecting the functionality of the user interface tab in my spreadsheet.