What does a parenthesized current rating on a switch mean?
They may refer to the inductive load switching current as these can cause a higher than normal voltage and cause a longer arc at switch off.  This will be less than the AC resistive load current that a switch or a relay can handle.

The data-sheet you indicate does not shed any light on the matter.  A similar short-form data-sheet (link below) for the same (or close) part suggests that they can handle 16A at full voltage but only about 4A when a horsepower rating is specified, this would imply that motors (often rated in power instead of current) cannot use the full current capacity of the switch.  This supports the theory that it is able to handle 12/16A resistive loads (filament lamps and heaters) and 4A inductive loads (motors, inverters and a lot of modern electronics with switching supplies) and is a common range of limits one can see in switches.

Most consumer mains AC switches that are rated for 125 or 250V AC are able to handle similar currents when switching DC but at about 10 to 20% of the rated voltage.  This is due to the fact that a DC arc does not extinguish automatically at twice the line frequency and needs a larger contact gap. Sometimes a second voltage may be indicated in parenthesis and this would usually refer to the DC rating, the current limits would obey the same kind of limit as the higher voltage mains ratings limiting the power the switch can carry to much less than what can be switched by the same switch on the mains side of a step-down transformer.

http://html.alldatasheet.com/html-pdf/402125/CWIND/GTS448B101A1HR/157/1/GTS448B101A1HR.html