A picklist is a dropdown list that provides users with a list of valid values to select from. Filtered picklists contain more complexity and reference other lists in a model: the driver and filter lists. Selections made in a driver list determine what users can select in a filter list.
If you want to create many-to-many filtered picklists, you can use the Assign action with a valid combinations list to map the relationship between list items in the driver and filter lists.
The Assign action enables you to easily assign a list item to a parent. The action requires a numbered list with at least one list-formatted property, and a parent hierarchy. For many-to-many filtered picklists, the driver list must be a parent of the valid combinations list.