Duality

It seems I haven't explained Duality well enough.

Duality is being able to fulfill two roles with one unit.

In 40k, that means anti-infantry and anti-tank.

Land Speeders are a great example.

They can be equipped with a Multi-melta, and a Heavy Flamer.

You usually won't fire both at one target, but you can if you want to.

Point is, the heavy flamer hurts infantry and the multi-melta hurts tanks.

Both are slightly effective against their non-chosen type, but they excel at their chosen tasks.

Almost all of the IG vehicles I inserted into this list here, have anti-tank (melta blasts) and anti-infantry (heavy flamers).

The blasts are also effective against heavy armored infantry--something heavy flamers don't really do well against. As another example:

A primary reason I run Missile Pod/Plasma Gun Suits in my Tau army is duality.

(And that there isn't anything that does their job in the army!)

I can take down light vehicles, infantry, and heavy armored infantry with these. Something no other suit config (or anything in the list excepting Hammerheads) can actually do.

In some cases, you can't get duality out of a unit. Broadsides and Valkyries are excellent examples of this.

Broadsides will never really do well at killing infantry, but massacre tanks.

Valkyries (the Carriers) will never do well at killing tanks, but massacre infantry.

Does not mean they can't, but they are far away from each other in the spectrum of what they can do versus the right type for them.

Some people will get this, and run effective armies. Some won't, and will run ineffective armies.

Duality is key to balanced armies. Keeping Duality in mind when building your lists will make your lists perform better.

Look through that Guard list again. Note how virtually everything in the army is capable of Duality? That's what makes great lists, having multiple layers of threat built into each unit.