When it ask for your Steam name, for a PC game event, is that your Steam username used to signin with or is it the one you can create in the community/Steam Profile bit?
Pretty much the same. But since Steam usernames aren't unique (unless that's been changed) it'd be best to link to your Steam profile website, so that the organizers can add the correct person to their friend list.
Steam usernames are unique, and not changeable; that's the one you use to log in.
You probably mean that steam profile names aren't, and that is indeed true.