REG-fast iputs and fast outputs

Hi to every one
i want to know what is fast inputs and fast outputs? and example for this. i heard digital I/O and Analog I/O what is the difference between Digital I/O and Fast I/O ?plz let me know

REG inputs are considered high speed inputs cause they can sample the change of state and do something very fast.

A typical example is on a servo drive where a home sensor or registration sensor is used to capture the encoder position at that moment of time.

These times are typically in the microseconds.

A high speed output is typically used for high speed pulse signals or to signal something with very little delay. Less than 1ms would be ideal.

A brief answer but enough to get you thinking.