This folder contains uploaded documents
This folder must be writeable (chmod 777)

By creating subfolders, the documents will be stored in these subfolders
(corresponding to the id of the user).
Creating subfolders is not mandatory;
it's recommended if you expect huge amount of documents

You can create subfolders by 1000 of id: like upload/1000/
You can also create subfolders by 100 id: like upload/1000/1100/

If subfolders doesnot exist, the files will be stored in the root folder /upload/
If you use subfolders, be sure that they are all writeable (chmod 777)