In common scenarios, there are linked lists that need to be included as hierarchical comboboxes in your forms.
For example a typical Country/City scenario would include 2 lists.
Countries (ID, Title)
Cities (ID, Title, Country) where the Country field is a lookup for the Country List.
In order to declare the comboboxes for these lookups, you must follow the normal procedure to declare the combobox lookups (list url, list name, display field, value field) and then for the City combobox in the Lookup Details tab, you must select the Constraint By field (it is the field in the current form where the Country is stored) and the Match To (it is the Country field inside the Cities list).
After configuration, the form automatically identifies changes in the constraint fields and reloads the depedent list using the criteria provided.
Additional criteria can be applied to the list query by using the "Lookup Criteria" property. Instructions on how to add criteria can be found using the following links: