After a quick Google search of "Substance designer alpha", it seems that an "alpha" may be some sort of mask, a .png, or .jpg.???

So what is an alpha???

- Brian

An Alpha seems to be a white on black background .jpeg or .png.
Or colored?
It is added, or imported to Substance Designer.

Transparency? Background is black/transparent?

Gimp can create a rounded rectangle Alpha...

Watched a youtube video...Steve's Tutorial, How to import custom alphas,
(Paint splatter vector)