I am looking at the specifications of an EMI filter for single a phase ac application, and I noted that the datasheet specifies an operating frequency from DC-400Hz? Do you have any idea of what this means?
The DC-400Hz operating frequency means that it's designed to allow power to pass through at any supply frequency in that range.
Most people will use a mains filter at 50Hz or 60Hz, the nominal operating frequency of a mains supply. However, it will also allow DC to pass though, so can be used to protect against EMI in DC operated systems. 400Hz supplies are rarely seen on the ground, but are common in aviation, where the higher frequency allows the use of smaller and thus lighter transformers, motors and generators.
As it's an EMI filter, it will attenuate signals at higher frequencies. The datasheet gives graphs of typical attenuation starting at 10kHz, though significant loss is only kicking in from 100kHz.