When an author selects Background Image, it dynamically sets a Pathfield field, as shown in this illustration.
Likewise, if the author selects another value from the dropdown (for example, Background Color), a second drop-down field is dynamically set, as shown in this illustration.
To develop Touch UI dialogs, you use the AEM Granite API.
This article walks you through how to develop this Dialog that dynamically sets fields. To read this article, click https://helpx.adobe.com/experience-manager/using/dynamic_touchui.html.
NOTE: To learn how to dynamically work with AEM Classic UI dialogs, see Dynamically updating AEM custom xtype fields.