BC 16 migration.json to migrate custom fields and tables to extension in upgrade

Starting in BC 15.3, Microsoft added the migration.json table that from their doc, looks like it will migrate custom and ISV fields and tables to pre-existing extensions using the extension app id.
If this works like this, it will make upgrades less expensive, since we do not have to write code to move the data moving to extensions, th eupgrade will do it for us.
The online doc is minimal.
My questions are:
1) Am I understanding this upgrade functionality correctly?
2) Does it only use table numbers and fields numbers since for ISV migrations, we have to add a prefix or suffix to every field that is not part of the older naming conventions. i.e. field 37,000,000 will move to the extension containing it even if the old field name was "My Field" and the extension field name is "ABC My Field", and same for table names - table 37000000 "My Table" will migrate to the extension as "ABC My Table".