I have so many different types of notes I will put on players that it'd probably fill a couple of pages here. This is just stuff I've developed over time (& is not confusing to me... instead quite the contrary).
Little things.... player's tendancies that you can exploit... ie. typical one.. player defends w marg. hands & tends to donklead out on most flops (along with this one... rarely double barrels if his donkbet is called... or always folds to raise.. is never donkleading strong hands, etc. etc. etc.).
Just part of one scenario, & some stuff I look for & take note of.
Some other things to get you going might be.... how do they play their draws (ie. strong draws... are they more apt. to check/call out of position, or do they lead out 'semi-bluff' with draws,... do they double barrel unimproved.... is villain capable of 3-barrel
bluffing, etc. etc..).
What types of hands are they flatting with pre in early levels (I look for weaker players... ie. 'Flushmasters' who are calling (out of pos. in pos... to them it doesn't seem to matter), with any suited broadway.. ie. they're calling off with J6s to an EP raise from a tight player. {you'll see some players who never seem to be able to fold J5s preflop}
Other things... stack size play. ie., does villain raise/fold pre w <20bb's Any abnormal bet-sizing that would indicate that villain is a weak player... and what those bets may mean.
The list goes on & on.... & on. I guess I could sift thru some of the ones I've actually put on a list & try to post a few of them (a few besides the few above)