Is there some way we could interpret differences of casual matches and competitive matches so that we don't confuse ourselves? that'd be great ^_^
Unfortunately the game doesn't save the match's options (well it even doesn't save the default match options!), so it's impossible.
But with BrawlStats you can start a session whenever you want. Explanation:
When you run BrawlStats it loads your NAND's Brawl savegame and check the game running time. If it's different from the last session it will ask you if you want to save it as a new session.
This means, if you are going to play casual matches and a competitive matches on the same day, you can create a new session after playing the casual matches. Then play again the game now in competitive matches, when you run BrawlStats you will create a new session (the same day, but a totally different session). The first session you created will contain the casual matches information, while the second session while have the competitive matches session.
Then you can write on a paper which sessions are casual and which are competitive. Another idea is to use different usernames when playing casual or competitive matches.
This is the only way I can think of having the information separated. There is no other way.
On the other hand, it could be possible to add an option to set any session as competitive or casual. But I'm not interested on it now, the session file format is already finished and I don't want to touch it anymore. And I don't think I'll modify it in the future, because that could break the old sessions and create compatibility problems. Sorry
Also... Maybe I'm wrong (as I never followed the tierlist history) but BrawlStats could be also very helpful on creating the famous tierlist