"User process" in this context is referring to the process running on the client machine that creates the connection to Oracle. In other words, if you are using Application A (SQL*Plus, TOAD, etc.) to connect to Oracle, the user process is SQL*Plus, TOAD, etc. If that user process dies while you were in the middle of a transaction, that transaction will be rolled back. This will happen as soon as PMON discovers that the client has died which may take a bit of time-- it isn't always trivial for Oracle to distinguish the failure of a user process from a user process that just isn't issuing commands at the moment.