The POSIX tag is for questions about the POSIX-standardized features for `ex` and `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
vi on servers or embedded devices which may or may not have
Vim, your questions about such should be tagged
When your question is specifically about the original
and its features/design, even when its features differ from the
modern POSIX standards, use the
original-vi tag instead.