Defining dependent picklists can help with better data quality. It allows you to limit the options of one list (dependent picklist) based on the selection of another list (controlling picklist).
See Salesforce help on defining field dependencies for more details.
In Propel, if you want to make a dependent picklist based on an Item's Category selection, you should also list the Categories in the Category Control field.
Go to Setup -> Object Manager -> Item Revision -> Fields & Relationships, and click the Category Control link.
Scroll down to Values and click the New button, list the category names that you want to be able to control with a dependent picklist, and click Save.
Then, set up the field dependencies as described in the Salesforce help link above.