For example, you could have bullets that will collide with the player but not with the enemy, and viceversa. So you can use 2 categories: 1(&B01) = Player bullet var PLAYER_BULLET=&B01 2(&B10) = Enemy bullet. var ENEMY_BULLET=&B10 SPCOL SPEBullet,1,ENEMY_BULLET If you want a bullet that collide with both the player and the enemy, then: SPCOL SPEBullet,1,ENEMY_BULLET OR PLAYER_BULLET