Many newcomers to Multics are confused by Multics' use of "segment" versus the more common, these days, "files".
The Multics Storage System (see Multics Storage System was built upon the notion of segments and pages. Storage system entries (segments, directories, and links) are used to help organize segments into a hierarchical storage system. Directories are actually implemented as special ring-0 segments.
In Multics, the language abstraction of file I/O is supported by the I/O system. Files in the I/O system can be stream or record oriented. The vfile_ I/O module (in the early days, file_) was implemented in the I/O system to access segments and multi-segment files.
In the old batch days, files did not have names. Just tape drive numbers. Your FORTRAN program read from "1" and wrote to "2". You filled out a "job request" with your input deck that told the operator what tapes to mount on what drive. Eventually, the notion of mapping between drive numbers and names was introduced, I think IBSYS and then OS/360 introduced the idea of a mapping between drive numbers and names, when persistent disk files were invented and then catalogued.
Despite the existence of "files", many commands and active functions (AFs) access segments, rather than files, for speed and simplicity. There is less bookkeeping and accessing segments is as easy as accessing memory, variables, buffers, etc. Text editors, compilers, system tools, etc use segments directly. Very few commands and AFs use files.
File access is used for:
There are two ways of accessing the content of "files" in the storage system: