In WordPress 4.7 the concept of changesets was introduced in the Customizer (#30937). To understand the new Customizer improvements in 4.9, you must first go back and review what was proposed and implemented a year ago: Customize Changesets Technical Design Decisions Changesets are a way to persistently store changes made via the Customizer framework. Changesets … Continue reading New Features and Enhancements with Customizer Changesets in 4.9