From what I see it is just a 0.5GB UMA buffer setting, similar to the already available 3GB, 4GB, 6GB and 8GB options.
The system has 16GB RAM. Some of that can be reserved for video memory. This is what the UMA setting is. This is called dedicated VRAM
The rest of the RAM will be managed by Windows, which can give up to half of it to the GPU. This is called dynamic VRAM.
So for the 4GB setting you will see 4G dedicated, 6G max dynamic VRAM in e.g. Task Manager. (Actual numbers are a bit smaller.)
With Auto UMA I am seeing 0.5G dedicated 7.5G max dynamic. So this is by all means looks like a 0.5G UMA setting, there is nothing more “automatic” in it than the other settings.
Is there anything more to the naming of this setting than marketing?