flight plans Flight Planning Files in MSFS

[yellowjack 28]

I have noticed that if I make a flight plan in MSFS I can save that plan as a file but I can save it in two different ways, on way saves a file with just the file name (in my case usually the Departure and Arrival ICAO codes). One way whilst the other way adds the suffix .PLN. I would be grateful if someone could explain the difference between these files and how they could/should be used. Thanking you in advance.