Understanding a common mode signal conditioning module
It means that the input amplifier is isolated galvanically from the output stage and therefore can have 1500 volts between circuits at the input and voltages at the output. In effect this does mean the common mode input range is around 1500 volts but this will have somewhat degraded performance as that voltage gets bigger and certainly the common mode noise rejection will be much poorer at higher frequencies. A link to the data sheet should help.