Voltage signal and Current Signal
Usually it has to do with what the author thinks is the best way to sense the signal that's on that pair of wires.  If they say "voltage signal" then they (probably) feel that it should be applied to a high-impedance amplifier that responds to voltage; if they say "current signal" then they feel that it should be applied to a low-impedance amplifier that responds to current.

An example of a current signal is a 4-20mA current signaling loop.  A device that uses this expects to be fed a wide range and possibly varying voltage, and it will impose a current proportional to the quantity it's measuring on the power supply.

An example of a voltage signal would be the output of a typical op-amp.  The op-amp imposes a voltage which remains largely unchanged regardless of variations on the current, at least up to the point where the amplifier can no longer coerce the output to be correct.