The Metadata can be used to get cascade variables/Values, which allows to fetch corresponding subcategory by pulling from its parent category.

Alternatively you may use data-list and lookup to implement, please refer to Lookup and Cascade(working).

Use Case

Step 1 - Create Metadata with cascade

Let's create a Metadata in Settings. please refer to Maintain metadata of your business .

In this example, we have a metadata named "Expense Type", and we will see the cascade from ① -->②-->③

Step 2 - Set Metadata in Form Designer

Go to Form Designer. 

Let's drag and drop three Metadata controls according to our example, You may add more cascades as required. All variables should be bound to the same Metadata. 

• The first is the Parent-category 1, which is no associated parent (unless you want to set it from its subcategory).Always the "Hierarchy" is off as default. 

• The second is the Subcategory 2, which should be associated with parent variable of "Parent-category 1";

• If more, then the third is the Subcategory 3, which is associated with parent variable of "Subcategory 2";

Then let's set the "Dynamic Display Rules" in subcategory 2&3, which makes a cascade view. 

Step 3 - Manage the layout and Publish

Preview and check the results before it publishes.

Furthermore

In some scenarios, you can also use it in list control/variable and get the same effect.