Roland SuperNATURAL BRASS ARX-03 Owner's Manual page 20

Stack Mode
This allows you to successively layer each instrument in the order
you've specified "Playing settings (Section Style screen)" (p. 29).
Using this mode allows you to simulate the bell tones played by a
brass ensemble.
When you play single notes
Only the first instrument will sound.
1
22
When you play a second note as well...
The first instrument will continue sounding, and the second instrument
will be added.
1
2
When you additionally play a third and fourth note...
The sounds of the third and fourth instrument will be successively
layered.
1
2
3
4