The ee203 specifications show guaranteed values for the normal operating range of 1.5V to 5.5V. However, there is also an absolute maximum rating of 12V. This causes some confusion about whether the unit can be used outside of the range of 1.5 - 5.5V.
The short answer is yes, but performance has not been characterized and is not guaranteed to meet specs. The lower voltage limit is around 0.5V with the upper limit constrained by the internal power supply to about 8V, although your mileage may vary somewhat due to manufactuing tolerances. The absolute maximum of 12V means that the unit won't blow up if you accidently hook it to 12V.
Calibration is locked out outside of the normal operating range, so if you want to use it out of spec then calibrate at 5.5V if you are using it above 5.5V, and calibrate at 1.5V if you are using it below 1.5V. That will help to ensure that your results are as accurate as possible.