For questions about external filters (:range!). Not for questions about the vimscript function filter (use vimscript) or the command :filter (use ex-command or vimscript)

An external filter is a program external to the editor that reads from standard-in and writes to standard-out (The filter pattern, Pipes, Redirection, and Filters, Rule of Composition).

:help filter says

A filter is a program that accepts text at standard input, changes it in some
way, and sends it to standard output.  You can use the commands below to send
some text through a filter, so that it is replaced by the filter output.
Examples of filters are "sort", which sorts lines alphabetically, and
"indent", which formats C program files (you need a version of indent that
works like a filter; not all versions do).  The 'shell' option specifies the
shell Vim uses to execute the filter command (See also the 'shelltype'
option).  You can repeat filter commands with ".".  Vim does not recognize a
comment (starting with '"') after the `:!` command.

A question should have if it is about the usage of external filters from a vi-like editor as defined in the Help center.

A question should not have if its usage of the word "filter" relates only to the function filter() (use ) or to the ex-command :filter (use or ).