Whether you play in a tournament or on one of the sites/apps above, your game may be scored under a particular scoring system. While 18 supply centers is the object of the game, it is actually rare for a Diplomacy game to be played to that conclusion, either because the position becomes “stalemated” (i.e. no one can achieve 18 centers) or because the players do not have (or want to take) the time needed for completion. Because of this reality, and in order to allow for multiple games over the span of a tournament or league season to be amalgamated into an overall rating, various systems have been developed for use.
Draw Sized Scoring (DSS) is used in some online games as well as at the Dixiecon tournament. Players achieve a higher score by reducing the number of players sharing in a drawn result. In the original version, called Draws Include All Survivors (DIAS), each survivor in a draw receives the same number of points, with some variations also added points per center as a tiebreaker. In more modern versions, survivors are able to vote themselves out of the draw (non-DIAS) but still receive lesser points for survival. The Dixiecon Scoring System is a non-DIAS version of DSS.
Place-Based Scoring is another popular type of system, in which the goal of the game, short of taking 18 centers, is to have more centers than other players when the game is called. While the number of centers is obviously key, it usually matters very little in such systems how many more centers a player has versus the next place in the end-of-game pecking order. A popular example of Place-Based is Carnage Scoring.
Lead-Based Scoring encourages the board leader to widen the gap between that score and rival players, by increasing the benefit of that lead with each additional center. An early version of this type was Sum of Squares, which rewards a large spread between the topper and all the other powers. A more recent version, which combines this effect with additional play incentives for the non-topping players is called Tribute.
There are other systems as well. Though you may end up preferring one over another, you should try to learn how to succeed regardless of which system is used.