Sub slot unit comes in to play when one of your main 3 units dies.

Only the commandment on the unit who is furthest left in your team is activated. Only one commandment at a time.

Does the "main" position provide the hero with any benefits? also is the only one commandment rule apply to pvp.

It only determines the heroes you want to use first. Then if one of the main heroes die, the hero in the sub slot will come into play. Now, some heroes placed in the sub slot can provide benefits to the main team using their unique ability. Here's the link if you want to learn more. And yes, the one commandment rule applies in all areas including PVP.

I see, thank you so much. I'm trying to make the most out of this game, so knowing little things like that matters a lot for me. The one commandment rule literally only shows up for a split second on a loading screen. I wish the small and large bits of information like this would be ready to read in the game itself in a sub section.

I love the wiki/fandom but I still believe the bulk of overlooked information should be in game to maximize play time.

True, that's why we have this community. We'll also compile all these bits of info so readers can access it in one page. You can always look back and ask the community if you have any questions about the game. Hope you'll have an efficient progress. Have fun!