Which slot you can use free spins for depends primarily on the respective offer. In some cases, you get general free spins from your online casino. These can usually be used for all slots that the casino offers. In other cases, free spins only refer to the slots from individual game developers, such as NetEnt or Microgaming. If so, the free spins can only be used on slots from that particular developer. There are also free spins offers that only relate to very special slots. In that case, for example, you get 50 free spins for a slot like Starburst. So you can only use these free spins for this game.