The iPad/iPhone storage is the local, on-device storage.
For security reasons, all iOS applications are restricted to a container known as a sandbox. Applications have unlimited access to their own sandbox, but they are not allowed to access files in the sandbox of any other application. This gives iOS users a great deal of peace of mind with regards to privacy and security, as it ensures that any malicious or malfunctioning software they download is not able to cause trouble beyond the data stored its own sandbox. This comes at the cost of making it difficult for applications to communicate or share files, for example Texpad and a file storage client.
Texpad has unlimited access to its sandbox i.e., its internal storage of documents. This storage is managed in the very first section of the Documents’ pane, entitled ‘iPad’ or ‘iPhone’ based on the iOS device the app is running on,