If that load is rated at 150W and it is designed for up to 24V system (assuming due to model number, DL24).
Then if the electronic load is PWM, then at max current, 100% duty, with 24V and 150W, it follows that;
I (current0 = Power / voltage
I = 150 / 24 = 6.25A
So if PWM, the load is pulsing 6.25A.
600mA will be a duty cycle of;
0.6 / 6.25 *100 = 9.6 %
The ZTX690B has these max pulse current rated as;
There appears to be no figure in the data for pulse duration and the load gives no info about what PWM frequency uses.