Synchronized JSON is a feature (added in version 5.1.5) which detects changes to local JSON files and allows the user to update the corresponding field groups within the Database. This functionality is visible when viewing the field group admin page but only when ACF has detected there are field groups available for synchronizing.
Available for sync
JSON field groups are determined ‘available for sync’ when either the JSON field group does not exist in the DB (field group key is used to match pair), or when the JSON field group contains a higher ‘modified’ value than the DB post’s modified date. The modified value can be found within the JSON file (assuming the file has been created by ACF PRO version 5.1.5 or later) and is a GMT unix timestamp from when the field group was last saved.
If you do not wish for your field group to qualify for ‘available for sync’, simply add an extra value to the JSON array called ‘private’:
'private' : true. This may become useful for plugin / theme developers who wish to keep their bundled field groups hidden from the user.