I would like to have additional compass units as a selective option, such as Mils, Gons, Nautical line e.g.
For working in my environment the NATO Mils are of particular importance. (6400° circle)
As @twolpert mentioned, you can change the compass units to MILS on the GPSMap 66i, but on the inReach Explorer+, inReach SE+, as well as older inReach devices, the compass units cannot be changed. To change compass units on the GPSMap 66i, go to Main Menu > Setup > Heading > Display > Mils. For more information about Heading Settings of the GPSMap 66i, reference the owner's manual.