PT100 temperature sensor: meaning of "measuring current" specification
If you look at the headings of those columns, you will see that these are maximum values. The datasheet is saying that if you exceed those currents through the device, then enough heat will be generated that some accuracy claims will no longer be true, since the device is heating itself. There is no lower limit (although at some point the voltage difference generated will be so small that you will run into noise, either due to the limit of your ADC, or thermal noise (and probably other noise sources)).