What is a 3-pole fan isolater and why would I need one?
It allows you to disconnect (isolate) the fan from the rest of the circuit so you can service it and leave the lights and other power on at the same time if needed.
Here are a couple of resources:
https://www.tlc-direct.co.uk/Technical/DataSheets/MK/3_Pole_Fan_Isolator.pdf
https://www.se.com/ww/en/faqs/FA166866/