How does the sensitivity setting affect the measurements made by an OSA?

This specification describes the different circuits that are available to amplify the photodetector signal. Each amplifier circuit has its own signal gain and response time.
 
A high sensitivity corresponds to a high-gain circuit with a relatively slow time response (long integration time), resulting in a relatively slow sweep speed. This setting is important for applications involving a very weak input signal.
 
On the other hand, a low sensitivity setting offers a fast sweep speed with a trade-off on the signal gain.