Q: Creating Luma Matte In Motion 5

I'm not quiet sure wether I understand your question correctly… (sorry, lost in translation )

Under Filters/Keying, you'll find a Luma Key

Or, if you like to use an existing clip as a key, use Image Mask to your to-be-keyed clip

Or, if you want to give an existing clip a b/w look (old Apple iPod campaign), try the manyfold color-correction tools

Aside from a Luma Keyer filter built-in...

Draw a shape that works for the area needing to be masked.

Set the Fill to a gradient, using only black and white; black blocks the image, white shows it, shades of gray fade between.

Right click the layer you want to Luma Mask, select Image Mask.

In the Inspector for the Image Mask, drag-and-drop the shape you drew into the image source well.

Set the Source Channel to Luminance.

Adjust as desired.

Or draw your shape, apply the gradient.

For that shape, in the Inspector, adjust the Blend Mode to either Stencil Luma (black hides, white shows), or Silhouette Luma (black shows, white hides).

All depends on how your Layer Objects are stacked up, and what exactly you're trying to achieve with it all.

Individually