How does OSM2PGSQL append flag work for updates?

I am wondering about how OSM2PGSQL work with updates using the append (-a) flag. I currently have POSTGRES database with OSM2PGSQL data in it and use updates using OSMOSIS and appending to the database using OSM2PGSQL.

My question is how does the append work in OSM2PGSQL when doing update? does it delete and update records in the database or only append new rows when changes are made? How does the update work for OSM2PGSQL using append flag with a change file? How does osmosis update differ from using osm2pgsql append? Is there a way to track the updates when appending?