[This section reads a bit esoteric - mea culpa. You can safely skip it unless you want to write a file system driver or maintain the TrueVFS source code.]

TrueVFS federates virtual file systems into a common file system space. To understand what this means, let's check the following definitions:

File System
A persistent container for file and directory objects which can be used in order to structure, store and retrieve arbitrary data.
Platform File System
Normally, the JVM provides an application with the means to access a set of one or more operating system file system types like NTFS or ext4. This includes network file system types like CIFS or NFS.
Virtual File System
A file system which is composed of one or more components which are not file systems by themselves.
Federated File System
A file system with a root directory which is a member within another (parent) file system. This definition is recursive, so the parent file system may be federated, too.
File System Space
The union of all file systems.

Note that the attributes virtual and federated are not mutually exclusive, i.e. a file system could be virtual or federated or both at the same time.

As an example for a file system which is virtual, but not federated, consider the HTTP(S) protocols: With their methods PUT, GET, POST and DELETE and the URI adressing scheme, the HTTP(S) protocols provide a subset of typical file system services.

As an example for a file system which is federated, but not virtual, consider a native Unix file system which is mounted at a directory path within the operating system's file system space, like e.g. /mnt/cdrom.

As an example for a file system which is both virtual and federated, consider any archive file which is accessed via TrueVFS's API: It's virtual because it's adapted to typical file system services by TrueVFS and it's federated because it's a file member within another file system - which is a native file system of the operating system in most cases.

Within this project, the terms federated virtual file system, virtual federated file system, federated file system and archive file system are often used synonymously. This simplification should do no harm because as a Java application, TrueVFS provides only federated file systems which are virtual file systems at the same time, too. The term archive file system is a legacy of TrueVFS's history which started as a small library for accessing archive files like directories. Of course, this is still true and in virtually all use cases a federated file system is persisted in an archive file.