The input range (or gain range) refers to the maximum and minimum voltage that will be digitised by the A-D converter. An input may be bipolar, covering a range from -50 mV to +50 mV for example; or unipolar, covering a range of 0 to 50 mV say. Many A-D converters have an actual range of 0 to 10 V, so an input signal with a range of 0 to 50 mV needs to be amplified by a gain of 200 before it can be converted. In Microlink systems you can set each input range from software.
You should always choose the range that most closely encompasses your signals, as this will increase the resolution. For example. Suppose the A-D converter has a 12-bit resolution. It dividesthe range into 2 12, or 4096, steps. In this case a 0-10 V range will be resolved to 0.25 mV, and a 0-100 mV range will be resolved to 0.0025 mV. Note though, that there is no point in trying to resolve signals below the noise level of the system: all you will get is unstable readings.