The ‘Drum set’ checkbox tells Dorico whether or not the options for drum sets on the Percussion page of Notation Options for voicing etc. should apply to this percussion kit.
That part makes sense, when it comes to working with an actual drum set.
But in a case where I have just a single instrument, or perhaps two (say snare drum and bass drum). Shouldn’t the stem directions apply even if ‘Drum Set’ is not checked? Or would ‘Drum Set’ always need to be selected to apply the stem directions?
When the ‘Drum set’ option is not set, stems will just follow the natural rules for stem direction, so if you put a note on a line or space above the middle line of the staff, the stem will point downwards.