I think posture, form and technique all go hand in hand.

I would refer to it as technique. Which is extremely important. In BB you will make sure you are hitting the intended muscle/muscles. In PL it will help you lift more weight. Plus it helps prevent injury.

Another way to put it is exercise vs movement. An exercise would be say on the lat pull down you move the weight down in strict form to your upper chest, pull back your shoulders and then control the weight on the way back up getting a full stretch.
A movement is where the person puts too much weight on the rack, straps in, and uses their whole body to move the weight. Looks as if they are parasailing. See a lot of this on plate loaded machines.

As Rusty mentioned there is strict and cheating movements. But they both have a purpose if you are intending to them and not ego lifting. They both have technique/form to them.

To me posture fall under the umbrella of good form

Most people think of posture as a fixed position. But knowing when to neutealise, pronate/extend/contract and to 'hyper' do these things under various/appropriate loads or at appropriate speeds, is the key to form and longevity / sustainability

Depends.. For example! My arms are always bigger measurement wise when I do heavy cheat curls!!! I try to sometimes take them out of the workout for fear of getting injured & do lighter, supinate, and squeeze like hell instead. But I always lose size going that route. Depends on the person I suppose... I think my fast twitch vs slow twitch fibers in my body make up prefers cheat curls for size when it comes to biceps.