Bazooka Joe and Sigrid were the two heroes in the original (now out of print) Dust Tactics Core Set. For a limited time, Dust-models.com offered a pre-painted pack of just these heroes in two different paint schemes. If you already own the original (now out of print) Dust Tactics Core Set, these would be considered an "upgrade" or an "accessory" because they are simply pre-painted versions of what you have. If you only own the Dust Tactics Revised Core Set, these would be considered an expansion because they add two new playable heroes to the game.