"Max down" is double the motion range of the first default setup, IIRC from tinkering. You can set a different value in other default sets, but when the user makes their own setup, the motion range is based on the first set.
IE, if your first set is a low-rider but your second set is really high, when someone has the 2nd set selected and clicks 'new setup', the user setup it makes will be the values from the 2nd set apart from the motion range, which will be calculated from the 1st set. (Again, only IIRC! It's been a while.)
Someone, somewhere, suggested using the first setup as a base to define that motion range. I haven't got to using that method yet myself, but it seems like a good idea.