The accuracy of an Electronic Distance Measuring Instrument (EDMI) is specified as $\pm (a \: mm + b \: ppm)$. Which one of the following statements is correct?

1. Both $a$ and $b$ remain constant, irrespective of the distance being measured
2. $a$ remains constant and $b$ varies in proportion to the distance being measured
3. $a$ varies in proportion to the distance being measured and $b$ remains constant
4. Both $a$ and $b$ vary in proportion to the distance being measured