Containers, categories and folders
When Remository was rewritten some years ago, there was a question of what word to use for groups of files. Categories had a particular meaning within Joomla, and Remository does not use Joomla categories.
Folders had a particular meaning from previous versions of Remository - they were the groups below categories.
For design reasons, it seemed a different word was needed. The one selected was “containers”. The main design consideration was that Remository allows files to be grouped together in a structure of arbitrary depth. It is comparable to the directories on a disk. Some are at the top level, but the work just the same as others that exist as subdirectories, down to any depth you choose. Remository is like that.
But sometimes the word “category” is still used with Remository, to indicate a container at the top level - one that has no parent. And the word folder is used, especially to indicate the container in which a file exists (whether or not it is at the top level). So in the administrator interface, if you list files, for each one a category and a folder are shown. They may be the same, if the file is in a top level container. But if the file is in a lower container, they will be different.
Note that the actual storage of files does not have to match the structure of the containers. The containers can be located wherever is most effective for the application. In the case of files stored in the database (see Remository Storage) the file storage is only related to the containers by the fact that every file records the ID of its container.