On a single system, the file system usually has the same structural characteristics throughout. For example, on a file system with a hierarchical directory structure, directories are permitted anywhere the access permissions allow. On a file system that is flat, i.e., does not have directories, directories are permitted nowhere. On a single system, the same structure semantics usually apply to all accessible files regardless of location.
However, there are exceptions in the single system case. For example, some systems are capable of mounting different versions of their file systems. A flat file system may have been used in a system's early development. As the system matured, a hierarchical file system became the norm. In order to maintain backward compatibility, the system is able to simultaneously access both the early and the later versions of the file system. In such an environment on a single system, the structure semantics would be different depending on whether the part of the file system being accessed was the early version, i.e., the flat version, or the later version, i.e., the hierarchical version.
Systems which access file systems with different structure semantics become much more common in a network environment. Client file systems may have structure semantics different from server file systems.
Different structure semantics often implies a different syntax for naming files. For example, a flat file system likely has no way of recognizing a pathname as specified in P1003.1-1990 since a flat file system has no concept of a directory. The cover illustration shows three different file naming syntaxes, i.e., DOS and Unix on the clients, and VMS on the server. Some mapping between the client's file naming syntax and the server's file naming syntax may be required.
Different structure semantics does not prevent the client from being able to have some level of transparent access. Among the objects in a file system that make up its structure are directories, file types, and links.