Determining the input current of an optoisolator based on output current and CTR
Think of the current transfer ratio of a optoisolation as something analogous to the current gain of a BJT. 

To use the device as a switch, the output current must be guaranteed to be limited by external circuit elements (current limiting resistor, etc) and not by current transfer being too low. Thus, one would use the minimum CTR for calculations, to make sure that even in the worst case, the output would switch hard when the input had the calculated current flowing through.



A rarer use of such a device is for analog signal isolation, e.g for low frequency industrial applications. In this case things become more difficult - apart from the obvious non-linearity of the device, of course. Designing for minimum CTR results in the possibility of too-high an output that swamps any subsequent stage. Designing for maximum CTR similarly results in the possibility of output being too low for some instance.

A compromise value partway through the CTR range is typically used, followed by some form of automatic gain control or manual trimming preset at the output to address device CTR variations. Also, wherever possible one looks for optoisolators or other mechanisms (transformers) with a more tightly specified CTR.