I've decided to use two SharePoint Lists for this example. One as the app data source, and the other for the dropdown list values. Using a SharePoint list for my dropdown values enables end users to modify the form logic on their own. You could use any data source you prefer.
The control values list (named: Impacts)
Keeping it simple, I created a list called Impacts with two columns as shown below. Title (main request type), and SCategory (aka sub category - I didn't want to use any spaces or symbols there). The list is sorted by Title, Ascending. (I keep these in a separate list so that I can always show the same set of values in my dropdowns, even if there is nothing in the datasource for my app related to these selections.)
The PowerApp List/Data Source (named: CR):
The SharePoint List that will collect the submissions from the PowerApp is very simple so as to facilitate this demo. It has three columns:
Change Impact (which is really just the Title column renamed)
Sub-Category (single line of text) - optional note: You could make this a choice list if users might be updating directly in SharePoint classic mode as well, only it can not be a multi-select choice list.
Answer (multi-line plain text field)
You'll notice I entered 1 item/record manually using the regular SharePoint "New Item" command, that's just to make it easier to customize my forms. (It can be difficult to customize the forms until you have at least 1 record in the list. You can always delete that configiration record later.)
Adding and Configurating the Dropdown Controls in 4 Easy Steps
OK. Now we're ready to have fun.....
Step 1: Generate a New Power App from the SharePoint CR List. PowerApps does most of the work for you, leaving you with a fully functional app with Browse Gallery, Display Form, and Edit/New Item Forms.
Step 2: Add another connection to the app (Content Tab>Data Sources>(far right of screen) + Add data source) that references the "Impacts" list from the site as well. You should now have two data sources in your app, the one PowerApps added for you (destination data source), and the one you just added to the other list.
Tip: You can reuse data sources for controls by dedicating a CDS entity, or other data source like SharePoint for that purpose. For instance, I have a SharePoint site collection that is open to "Everyone except external users", where I keep a bunch of controls stuff - but CDS is perfect for this too! Just remember to permission the shared data source so that all users have the rights to read (but not write), and when using SharePoint also disable search results on each list so that the items don't come up in enterprise search results. Aren't you lovin' the fact that you can add additional data sources to your SharePoint PowerApp views so easily!?
Step 3: Make some room above the gallery on the first screen and add a Dropdown Control (Insert>Controls>Dropdown), I named mine "ddSelectType". Set the "Items" property to Distinct(Impacts,Title) to get all the main topics from our Impacts list into that dropdown. Run to test (F5).
Step 4: We're almost done already! Let's make some more room and add another dropdown, which I named ddSubCategory. We will simply filter the ddSubCategory dropdown based on the selection from the first dropdown.
The Items property of ddSubCategory:
Filter(Impacts,ddSelectType.Selected.Value in SCategory)
Run and test! I just works!
I plan to share many tips on making your apps interesting, and incorporating data validation. Let me know if you'd like any particular topic and I'll do my best to make that happen for you! Keep visiting the Community because I'll be posting video demos with your popular questions asked as well.