Hello Jens,
From the JavaDoc for abortScript(...) (underline is mine):
Exits the script and optionally aborts the transaction. Abort the transaction via this method when theend userhas made an error. If the error is due to a bad configuration - that is, something the administrator or the solution can and should fix, then use badConfiguration() instead. This method does not provide details to the end user about the trigger script. We assume that the user does not need such details if the problem is in the user's data rather than in the script or configuration. This script does not write to the log. If you need to know track whether a particular error occurs often, augment your script. If veto is false then the script is exited, the transaction is allowed to proceed, no message is presented to the user or written to the log, and nothing shows up in the server log.
This always aborts the script. When the boolean value is true, it also aborts the entire transaction. The warning message is only shown if the entire transaction is aborted, because currently Integrity can only provide a message to users as a result of processing, and that only happens for sure when the boolean value is true.
If you would like to be able to display messages to users without affecting command execution flow, or without having a result of processing, please attach your organization to CS83676: How to display an information dialog from a trigger (RFC 97577).
I hope that clarifies the difference--I had to check, myself.
Regards,
Kael