Chapter 7 is a liquidation, chapter 9 is only filed for municipalities, chapter 11 is for large-scale reorganizations, chapter 13 is for personal reorganization and chapter 15 is for the recognition of a foreign bankruptcy in the US. The bankruptcy code ends at chapter 15, so what is a chapter 20?

A chapter 20 is a chapter 7 followed immediately by a chapter 13 (7+13=20). The time restrictions on filing cases (8 years between ch. 7s, 2 years between ch. 13s, etc.) only applies to cases where you receive a discharge in both cases. In a chapter 20, you only get your discharge in the first case. The second case would typically only pay arrears on secured debts or priority debts (child support, taxes, etc.)

One of the main reasons for doing a chapter 20 is if your debts exceed the debt limits in chapter 13. The chapter 7 eliminates the debts so you get below the debt limits while still allowing you to use the chapter 13 to cure arrears on long-term debts.