You can define relation between fields on Action Launch Forms - decide which fields should by reload when value of the field was changed. 

Fields selected in refresh parameters list will be reload after change of the value of the field. Lookups used in the selected parameters will be recalculated to match current form fields values. The functionality can be used to build cascade field relations.

Example: In Refresh Parameter section for field 'Category' 'Items' parameter is selected. 'Items' field value and title depend on 'Category' parameter value. After change of 'Category' field 'Items' field is recalculated and reloaded to mach current selection. Also 'Selected item' field depends on 'Items' field value and is selected in its 'Refresh Parameter' section.

Additionally check our walkthrough how to use Form fields interaction with provided example.