What is .uptnl file in package?

I know .umap,.exp and the likes. Never heard of uptnl. My packaged build throws error saying it misses one. Looked in source and found it here and there, but no details anywhere about what it is.

I have exactly the same problem with chunking.
It create optional pak containing the .uptnl extension.

But the file does not exist in the folder.

Did you find why there is a reference to this kind of file?