Polyphonic Sequencer

Hello

im working on a polyphonic midi sequencer (for drum patterns mainly). i just finished my JSUI part of it and during the next days i will try to implement my midi engine .
i want to sequence multiple rows that contains midi notes . rows stands here for polyphony . where every row of notes has its own thread .... (?).
The question is :
Do i need to execute every row using [poly] ? or im allowed to do it without ? im wondering because i saw some chorders that plays couple of notes simultaneously , with different velocities and durations . i cant find it but i swear i saw it .

:)