Speck Electronics ASC - Manual (Page 23)

Because the ASC uses an external power module, the chassis is not
automatically earth grounded. If you experience hums or buzzes, this
screw may be used to connect the chassis of the ASC to known earth
such as an AC receptacle cover plate.
This connector is available to provide a source of dual regulated DC
voltage to an external device. This connector is reserved for future
Speck products.
23. Aux DC I/O
22. Chassis Ground
19 Chapter 3 Operation Section