In AFF4 terminology, a Volume is a single file which contains other AFF4 objects within itself. The Volume itself is an abstract concept. AFF4 defines a number of standard implementations for AFF4 volumes, the most common of these if the ZipFile volume which is based on the popular Zip archive format.
Since AFF4 objects are defined both in terms of their information model, and their data, Volumes typically contain both metadata and actual data. We term a Segment as an atomic piece of data that can be stored in a volume. Segments are also AFF4 objects and therefore also have globally unique URNs.
A repeated pattern with the AFF4 design is that it talks about AFF4 objects with globally unique URNs. However, these objects represent and refer to a real file with a filename.
Volumes may choose to store and represent the abstract AFF4 object’s URN in a suitable way appropriate to the storage media. Therefore there is always a well defined mapping between the abstract URN and the concerete name.
For example, if the Volume chooses to store AFF4 objects as files on the filesystem, the URN may need to be escapes to remove characters which are not allowed within file names. This escaping scheme (although well defined) is merely a representational constraint of the AFF4 volume. We still talk about AFF4 URNs in their abstract form, and then locate them with in the volume.