update-database vs upgrade-database

I am trying to understand the difference between these two odacli options: update-database and upgrade-database. We are in the process of applying the ODA 19.18 patch bundle to our ODAs. We want to update a subset of databases from 19.17 to 19.18, rather than update all the 19.17 databases at once to 19.18. I know I can use update-database to update each individual database one at a time (after running a prepatchreport which takes a minimum of 25 minutes, and orachk always fails for various reasons, and therefore we have to always run update-database with -f option).