Programmable apparently:
to change the TDS factor value: after setting the temperature unit, press ON/OFF once to show the current value (e.g. 0.50 CONV). Select the desired value by using the SET/HOLD button, then press ON/OFF twice to return to the normal measurement mode.
via http://www.milwaukeeinst.com/site/db/doc/manEC59_EC60_ENG.pdf
It appears to provide both TDS and EC by switching modes. You can also determine the conversion factor by first measuring a solution with the EC mode and then measuring the same solution in TDS mode (PPM). Divide the TDS by the EC readings = conversion factor.