I can't discriminate between "measuring span" and "measuring range" of measurement device. Please help.

Answer 1

If you are looking at a current (for example) that can vary between 0 mA and 21mA that has a measurement range of 21 mA (21-0). The measurement range is how much the variable can change.

For that same current example the measurement span can be from 4 mA to 20 mA so that means you expect to get a span of 16 mA (20-4) read between those values. Anything more or less is considered out of range. The measurement span is how far apart the end points on your meter must be set.