Is this a Premiere Elements question? It looks like you are working with After Effects.
I work with Elements Premiere 2022. I am not sure if i have to export the Alpha Map separately or if it have to be inside the Picture File.
The Files are jpg. Maybe i try Tif or PNG. But i need to know wich Keyfilter i have to use.
After a small Chat the User said that the Frames with Alpha Masks are rendered Separately.
I cannot Manage to Setup two Diffuse Layers and a Alpha Mask in a Project.
I cannot Save the PNG with Alpha for a Animation. I rendered them in JPG and the Masks in PNG.
I use Cinema 4d r21.
I tried all Key Filters for Alpha but none is working. Premiere Elements has a whole different Setup.
The Support for Single users does not work because i cannot Register Elements.
Elements is not cut out to do this kind of editing.
You need a more professional program.