The lines between what is a main and what is an off dps are a bit less clear at least to the public than supports/healers or tanks
This might not be the opinion of everyone but it is mine, so it might be somewhat controversial.
My idea of a main DPS would be thats main purpose, and whose main weapon is primary fire.
So heroes like Junkrat, Widow, Hanzo, Mccree, Ashe, Soldier, Bastion, Reaper and Symmetra would be Main dps
Their main functionality comes from their primary fire, Junkrats bombs, widows shootypointygun, hanzos arrows.
Mccree has his shootytootymcpooty gun, same with ashe, and soldier, bastion is just a war machine, same with reaper mostly
Symmetras main way of killing people would be the noodle beam
Off dps would be where their functionality comes from within the whole kit moreso than their primary fire
Heroes like Mei, Genji, Sombra, Torb, Tracer, Pharah And Doomfist
Mei and Sombra are more supportive dps than damage dealing ones, as a large part of their functionality comes from something that benefits the team more than themselves.
Genji, doomfist, and pharah have kits that allow their primary fire to do more damage, and for them to find kills, genjis mobility allows him to get close which is his effective range, doomfists abilities do 90% of his damage. Pharah uses her abilities like genji, but just to stay up.
Torb is between main and off dps, but as id say most of his damage comes from the turret PLUS overclock, he fits better into the off dps
So, in short.
Main DPS: Their primary weapon is their primary fire
Off DPS: Their primary weapon are the abilities they have

