I just booked some train tickets through the Deutsche Bahn website (Munich-Fussen). What is the "BUS 818", "BUS 219"? Is that an autobus, or a different type of train than the RE or the ICE?
Probably means just a plain old Bus.
There is no train called a BUS. Those numbers sure do look like "autobus" route numbers, but not for any buses around Füssen. RVO (Regionalverkehr Oberbayern) runs the buses in most of southern Bavaria, and their website does not show a bus 818. There is a bus 818 in the Nürnberg district. Also, you don't need a bus to get from Munich to Füssen.
Where are you going on these buses?
Those must be buses in Munich. I did find a bus 219 feeding into the S-bahn station at Unterschleißheim. I haven't found bus 818, but there are also some 800 series bus lines in Munich.
A bus on the DB website is an autobus. I looked up the schedule for trains from Munich HBF to Fussen and found only 1 that required a bus (the numbers were not the same listed in original post). Did you spell Fussen correctly for the DB site? (ie with an accent mark over the u) If you can't generate german letters, spell it Fuessen on the DB website.
It is not an "accent mark" over the u. It is two dots (ü) called umlaut (modified sound). To generate u-umlaut on a PC, hold down the alt key while typing 0252 on the numeric keyboard. ö is 0246; ä is 0228. However, ue, oe, and ae are also recognized by the Bahn website.