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Legal and expected - for "online/daily" AKA what the rest of the world calls Correspondence Chess - pretty much any research except not asking others for help or, of course, using an engine.
Well... the Daily or Correspondence Chess is, or rather became way of getting accustomed with game, when time frame will start sliding down the quality of game will also decrease. So, as long, in the critical position player is on his own it seems to be quite good and fair way to enter the world of chess, besides how would you enforce any stiffer rules?
Now that game explorer is linked directly to the "slow" games it is obvious that it is allowed and probably common. Nevertheless, after a number of moves games enter into uncharted territory and there you are. If you moved blindly with no idea what the strategy is supposed to be, the middle-game won't go so well.
Several years ago in this topic, someone posted this:"The only type of assistance allowed is human vs human game databases for opening lines in Turn-based Correspondence-style Chess and Vote Chess, Chess.com Opening Explorer, chess books, and an analysis board. "
They appear to be quoting one of the rules. If indeed this is the chess.com rule, then it is considerably different than postal correspondence chess, which I used to play rather a lot. In postal chess, if I have a book by Daniel King and a game I am playing happens to arrive at a point where GM King states, "White's game is already lost" and that position is part of an opening database (online, MCO, whatever), I am perfectly fine having used that reference, and hence GM advice, to have arrived at a nice middlegame.However if the rule is as stated in the quote, then I am not fine using that reference on a chess.com "Daily" game, and by rule, am "cheating." When I played postal chess, I used whatever reference I could find. Once I played a game all the way through to a complicated queen trap once played by, IIRC, GM Michael Rohde, from a game I found in Chess Life. My opponent followed the losing line all the way to move 24 or so.
I have a moderate chess library, game collections by Mikhail Tal, Boris Gelfand, etc., tournament books, opening middlegame and endgame theory, and over twenty years of Chess Life. YouTube is crammed full of instructional video (any of my opponents are invited to use Backyard Professor as a source and I won't complain!). Unless the rules of chess.com explicitly allow these references, I would advise caution in using anything but a simple opening database. If I'm wrong, I'd love to see clarification, because I am quite a good bookworm, and don't want to assume without proof that what was good for postal chess is good for "Daily" chess.