The Site tag is usually the city (and region) and country where the game was played. The standard does not deal with correspondence games, so "?" shall be used. Obviously, new tags like WhiteSite and BlackSite would be useful.
After the game is finished, the following two tags must be changed.
The WhiteNA and BlackNA tags could be used. NA stands for Network Address, and could be the e-mail address of the player.
The TimeControl tag telling the number of moves in a time period is often useful. Unfortunately, the time period is given in seconds, so the following value does not look very intuitive: "10/2592000" (10 moves in 30 days, i.e., 30 * 24 * 60 * 60 seconds).
After the game, the PlyCount tag could be used to denote the number of half-moves of the game, which is useful when searching for games of certain length. For example, if the last move of the game is "15. Qh7#", the value of the tag would be "29".
Comments are written within curly brackets. They do not nest, and their internal format is not specified; e.g., there is no standard way to write complicated analysis in comments. I have used parentheses and semicolons to separate different variations.
3. c4 dxe4 {3... c6 4. Nc3 (4. e5!?) Bb4; 3... c5!?}
The following suffix annotations can also be used outside comments, appended to the move in question; e.g., "Qh7#!". NOTE: this is against the standard, but the PGN readers I have seen seem to accept this.
! good move !! excellent move ? bad move ?? blunder !? interesting move ?! dubious move