What it does

Every material in your scene renders as one flat block of colour. Open that image in Photoshop, magic-wand a colour, and you have selected everything using that material — the way V-Ray's MultiMatte and material ID passes work.

Mechanically it is a Blender AOV. Each material gets one AOV output node carrying a constant colour, and the view layer gets a MaterialID pass. That makes it a first-class render pass: it appears on the Render Layers node and inside a multilayer EXR next to Combined, with no second render and nothing added to your compositor.

It is also, deliberately, a checkbox. There is no build step and no per-material work.