The "update files only" contain the files that have been changed since the last update, so that people that already have the full version of the previous update don't have to re-download the full game again. But it ONLY has those files, so skipping an update when using them will result in missing files and can cause issues, therefore never use them if you don't have the previous game version. The version number for which the updates files are is always mentioned in the file name.
I have a video about using update files and how to move saves when downloading the full game again here: