Before NAV 2013 R2 the “database schema synchronization” was an unknown term. Either you could do a database schema change, or you couldn’t. There was no such thing as data loss. If you wanted to change a field type or delete a field (or delete the whole table for that matter), and there was data in that field in the table, you got an error. Data present – no go.
However, with multi-tenancy, and the possibility to mount tenant databases originating from different applications, suddenly there was an issue – how should NAV handle possible differences in database schema for different tenants? Should we force the change at the cost of data loss? And thus the term “database schema synchronization” entered the vocabulary of regular NAV Joes.