Article: The Arena Football League briefly had a two-game exhibition season in the early 2000s, a practice that ended in 2003 with a new television contract. Exhibition games outside of a structured season are relatively common among indoor American football leagues; because teams switch leagues frequently at that level of play, it is not uncommon to see some of the smaller leagues schedule exhibition games against teams that are from another league, about to join the league as a probational franchise, or a semi-pro outdoor team to fill holes in a schedule.

Question: What type of league commonly plays unstructured exhibition games?
indoor American football leagues