(a) Credit the starting pitcher with a game won only if he has pitched at least five complete innings and his team not only is in the lead then he is replaced but remains in the lead the remainder of the game. (b) The "must pitch five complete innings" rule in respect to the starting pitcher shall be in effect for all games of six or more innings. In a five inning game, credit the starting pitcher with a game won if he has pitched at least four complete innings and his team not only is in the lead when he is replaced but remains in the lead the remainder of the game. (c) When the starting pitcher cannot be credited with the victory because of the provisions of 10.19 (a) or (b) and more than one relief pitcher is used, the victory shall be awarded on the following basis: (1) When, during the tenure of the starting pitcher, the winning team assumes the lead and maintains it to the finish of the game, credit the victory to the relief pitcher judged by the scorer to have been the most effective; (2) Whenever the score is tied the game becomes a new contest insofar as the winning and losing pitcher is concerned; (3) Once the opposing team assumes the lead all pitchers who have pitched up to that point are excluded from being credited with the victory except that if the pitcher against whose pitching the opposing team gained the lead
continues to pitch until his team regains the lead, which it holds to the finish of the game, that pitcher shall be the winning pitcher; (4) The winning relief pitcher shall be the one who is the pitcher of record when his team assumes the lead and maintains it to the finish of the game. EXCEPTION: Do not credit a victory to a relief pitcher who is ineffective in a brief appearance, when a succeeding relief pitcher pitches effectively in helping his team maintain the lead. In such cases, credit the succeeding relief pitcher with the victory.
I believe it is totally subjective and there are no objective standards that I know of....