I haven't been able to find any info on this. I imagine documentation is in the works.

Yes, documentation is in the works, but in short it works like this:

In 5.7, if your theme supports a grid container, and you enable that grid container on a particular area, any block you add to that area will get that theme's grid container wrapped around it (unless the block type is set to ignore that grid container.) This only happens on areas where this has been enabled, and will only work if your theme is using a grid that concrete5 understands.

There's no backward compatibility issue for this, but for theme's that do support it, it basically means you can have a full page width area and have all blocks added to it intelligent wrap themselves in a container – but keep SOME blocks outside the container. This is useful for full page width image sliders, parallax image sliders, the horizontal rule block that we've included in the core, etc...

(The HTML block also ignores this container too.)

thanks Andrew... I am just tinkering with my own theme, I take it we place any add on themes into the application>themes folder as before.

But I am struggling in finding where I place my additional blocks as the application folder does not contain a placeholder or is this different now.

cheers