Bathroom Fan with Humidity Sensor Wiring Question
Presumably the fan and humidity sensor can be switched independently to give you a choice of operational modes. Switching on the humidity sensor gives it control of fan cycle. Switching on the fan directly overrides that mode. 


  The humidity control and fan can be operated separately. Use a 1- or 2- function wall control. Do not use a dimmer switch
  to operate the humidity control. 


You can connect the brown to a switch or you can connect it directly to the hot conductor to have the humidity sensor operate full-time. It doesn't really matter as long as both blacks and the brown from the fan unit have power when you want it to run.