Confusion indeed.
You are confusing tracks, trigs, and midi channels.
Quick answer : just set your midi tracks to midi channels 9-16.
There are 8 audio tracks (1-8) and 8 midi tracks (1-8).
With default Track Mode, Grid recording off :
Trigs 1-8 trigger Play Free tracks*
Trigs 9-16 trigger audio tracks, or Midi Play Free Tracks in midi mode.
Noob questions
Default audio tracks midi channels are 1-8. You can set them to off (PROJECT > MIDI > CHANNELS), or set MIDI > CONTROL to INT (notes and CCs).
Midi tracks channels are set in their respective SRC / NOTE SETUP, in midi mode. You can choose any midi channel (9-16 or 1-8 if you turned audio tracks channels off).
Just to add confusion, audio tracks don’t send midi with the sequencer (midi is sent only with knobs or chromatic mode), and beware of that :
15.2 HOW MIDI IS ROUTED
If an audio track and a MIDI track share the same MIDI channel, the MIDI track will block the audio track from sending out data while the audio track will block the MIDI track from receiving data
PLAYS FREE controls the behavior of the tracks. When activated the chosen track will be disconnected from the sequencer. When [PLAY] is pressed the track will not start playing. To start the playback of for example audio track 1, it needs to be trigged manually by pressing the first [TRIG] key. To trig MIDI track 1, press [TRIG] key 9 while in MIDI mode. Disconnected tracks will be stopped when [STOP] is pressed. Read more about track trigging in the section “TRACK” on page 81.
So yes, it is confusing, reading the manual carefully is highly recommended.