Microsoft's USE guide
says this: (emphasis mine)
Cases which require reordering should use the 'pref' feature to identify the glyph or glyphs that is or are to be reordered. Rather, the font designer is expected to use the 'pref' feature to signal when a glyph belonging to this class should be reordered through the application of the pref feature.
What does this mean? How do you use an OpenType feature to "identify" a glyph or send a "signal"? Do you really signal to the shaper that it "should" do the reordering for you?
Would it be more accurate to say "Prebase medial consonants (CONS_MED_PRE) do not get reordered automatically by USE. If you need this, do it yourself using the 'pref' feature" - or am I missing something really clever?