There are many different forms of annotation:
Variations – alternative continuations that can be replayed on the board.
Text commentary – embedded in the game (e.g. “This is the critical position.”).
Symbol commentary – in the form of special chess symbols inserted before or after a move.
Graphic commentary – in the form of coloured arrows and squares on the chessboard.
Training commentary – with training questions, answers, points, times, help, etc.
Correspondence chess commentary – to automate the management of correspondence chess games.
Critical position – critical openings, middlegame and endgame positions.
Pawn structure and piece path – to highlight the pawn structure or graphically display a piece path.
Medals – a special award for best game, tournament decider, model game, novelty, repertoire, strategy, tactics, etc.
The annotation palette is a helpful tool for quick entering of symbol commentary and variations.