Alternate Event Numbers can be assigned to each and every round if you wish. This means a prelim/final event could be event number 15 as far as the computer operator is concerned, but might be event 20 for prelims and event 40 for finals as far alternate numbers are concerned.
To set-up alternate event numbers, click Events / Sessions and click on Auto Number to automatically renumber all events in a given session with optional alternate event numbers. Or you can edit the alternate event number column and put in anything you want from 1A to 9999Z. The computer operator would still use the original event numbers, but for the Meet Program and other reports you have the option to use the alternate numbers. But to use alternate event numbers, first check the box for alternate event numbers in Set-up / Report Preferences.