This article is from the Amiga CD32 FAQ, by Stuart Tomlinson with numerous contributions by others.
A: Jumper number... 1 OFF = If you have 1 or 4 megs. ON = If you have 2 or 8 megs. 2 OFF = If you have 1 or 2 megs. ON = If you have 4 or 8 megs. 3 OFF = If you have memory. ON = If you have no memory (or defeat autoconfig). 4 OFF = Normal boot time. ON = Add 10 seconds to boot time (for slow HDs). 5 OFF = If keyboard has two Alt keys. ON = If keyboard has no Right-Alt (Right-CTRL used instead).