Question: I see in my cache directory sub-directories that end in .u, such as /category.u/; what are those?

Answer: The /category/ sub-directory will be for any cache files for your category archive pages (such as

The /category.u/ sub-directory is for any user-specific cache files for category archive pages. It sounds like you have Logged-In User caching enabled (WordPress Dashboard Quick Cache Plugin Options Logged-In Users) and this is set to Yes, and maintain a separate cache for each user.

This means that when a logged-in user visits a category archive page, they will have their own cache files generated in a /category.u/ directory; the cache filename for the user will be [user-id].html, where [user-id] is their WordPress User ID, e.g., /category.u/6.html, if their User ID was 6.