The POSIX tag is for questions about the POSIX-standardized features for `ex` and `vi`, which differ occasionally from the "original-vi" implementation.

The POSIX tag is for questions about the POSIX-standardized features for ex and POSIX-standardized features for vi, which differ occasionally from the "original-vi" implementation.

When you wish a solution to be portable to a wide variety of Unix or Unix-like systems (RHEL/CentOS, Ubuntu, macOS, Solaris, et. al.), the closest thing to a universal guideline for portability is the collection of POSIX (Portable Operating System Interface) standards.

For instance when scripting in-place file edits with ex which may be run on a wide variety of systems, or when intending to edit files with vi on servers or embedded devices which may or may not have Vim, your questions about such should be tagged POSIX.

When your question is specifically about the original vi program and its features/design, even when its features differ from the modern POSIX standards, use the original-vi tag instead.