Cascaded Selection helps you to set options connected to another Selection field depending on what is selected previously.
To set cascaded selections for a selection field, when you're in the Design Mode, simply choose Selection as the field type, and check the "Set Cascaded Selections" box.
A pop-up menu will display all options of the reference Selection field, in which you can add the cascaded selections choices, with each choice written in a line.
When you are done with adding cascaded selection options, don't forget to click on the Save button at the bottom of the window.
It is possible to set cascaded selection that refer to a field with cascaded selections, so you can have many levels of selection depending on categories.
Note. If you want to display either your cascaded parent field or child field on the listing page, you will need to select both onto listing page and use hidden attribution to the one you don't want it to be shown on the listing page.