I've been looking at the jumper settings on my carPC mobo...

There are 3 options that I am not sure of,

They are constant "constant 3v signal" and "constant 5v signal" enable and disabled.

What is this used for?

The other is this one.
J6 - Power Connector
1 5v SB
2 Ground
3 Ground
4 PS-ON#
5 (+) 12v
6 (+) 12v

On this one I assume is for external 5v or 12v, but what is "PS-ON#"