New Features and Enhancements with Customizer Changesets in 4.9

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: 2016-11-15: Updated the description for how and when setting changes are written into the changeset, and also how the iframe window is loaded during a refresh when there are pending changeset yet to be written into the changeset. For more see comment. Also included information about storing modifying user with each setting change, among other … Continue reading
Changesets are a way to persistently store changes made via the Customizer framework. Changesets contain the pending changes for any number of settings, and a setting can model any object in WordPress—whether options, theme mods, nav menu items, widgets, or even posts/pages and their postmeta. Changesets are identified by UUID (which is the post_name for the customize_changeset post type that stores the data as JSON in post_content). When a request is made to WordPress with the customize_changeset_uuid request param—whether to the frontend or to the REST API—the Customizer framework will
Source: https://managewp.org/articles/16504/new-features-and-enhancements-with-customizer-changesets-in-4-9

source https://williechiu40.wordpress.com/2017/11/04/new-features-and-enhancements-with-customizer-changesets-in-4-9-2/