Hmmm, I have a bunch of strips set to have unique directory for proxies but the group of strips keep writing over each other. So that every strip just uses the same proxy. Is it a bug or do others see this behaviour?
The option Proxy Custom Directory allows you to render image sequences for meta strips and scene strips into a single folder. They automatically have unique image names so seperate folders don’t need to be generated by Blender.
The option Proxy Custom File allows you to render seperate jpeg movies for each movie strip, in this case Blender will generate a unique folder for each strip.
If you accidently reverse the selection Blender will write the movie strip proxy files over each other.