Dependent Fields
This module allows to create a so called "#dependent #fields". I.e. a dropdown-like fields in which the available options depends on a selection in another field. A very common example is a Category->Subcategory. So when a certain #Category is selected - the #Subcategory field shows only options related to a selected category. There could be a lot more use cases for dependent fields such as Country->State->City or City->School.
Any customizable #form in UNA can be extended with this module and have dependent fields on it. I.e. Persons, Organizations, Posts, Discussions - literally any form can have dependent fields with this module.
Note that the module affects only the "select" type of fields (dropdowns).
You might be also interested in Conditional Fields.