These features was added to the master site synchronisation in August 2018. Please refer to the Master Site Synchronisation guide for basic settings.
The new things are:
“Show List Sync Customization” have been added to Mastersite Synchronization.
This new section enables you to customize how individual lists are replicated from the Mastersite, when you run a manual Master site synchronization.
The original settings still apply, and you should just select the options want to apply for the Mastersite sync you want to run (replicate webparts, features etc.).
If you only want to replicate one entity or from an item, you can write an Entity ID in the top textbox and use the appropriate buttons (Synchronize item, Synchronize from item).
You can use the new replication options by expanding the “Show List Sync Customization” section.
Now you should be able to see all the visible lists on the Mastersite. You can select or unselect the individual lists for replication, and you can select/unselect the individual settings you want to apply for the Synchronization.
When you have selected the settings you want, you can run the synchronization you want.
Note, the options to “Synchronize item” and “Synchronize from item” are only accessible when you have entered an appropriate Entity ID in the top textbox, though you can always run a “Synchronize all” sync.