EMH installs a lot of Mitsubishi systems. 99% of all of our systems have the MHK2 thermostat. That thermostat has multiple fan settings that are helpful in making your house more comfortable under certain conditions.
We recommend when it is mild and you’re just trying to keep your house comfortable in other words not the extreme hot or cold days of the year that you leave the fan on auto. The fan in the auto position will track with the modulation of the inverter heat pump And very the Airflow depending on the temperature in your home and outside.
The systems were designed to use high speed fan on the most extreme days of the year. When we do your load calculation we designed the system size based on the air needed for the hottest and coldest day. That Airflow requirement is usually closest to the high fan speed. So if you wanna get a little bit more umpf out of your system switch that fan to high. But remember when you switch it, it may run when the heat pump itself is off. 
It is good to run the fan as much as possible as 99% of all the systems we install have a high-performance filter. Anytime the fan is running it is cleaning the air. It is extremely important to keep the filter clean please see the changing the filter in your heat pump video.