According to dictionaries, scope and topic are
distinct concepts. Eg.

https://en.wiktionary.org/wiki/scope noun, sense 1.
https://en.wiktionary.org/wiki/topic noun, sense 2.

When a mailing list is set up, the administrator decides the scope.
Eg. "Concerns of Devuan users."

The topic of a thread is the subject specified by J. Doe when the
original message is created.

So a thread can be out of scope. Advice about a snag in MacOS is out
of scope for DNG. As long as the thread continues with the original
topic (Subject) it is "on topic".

A message can be "off topic" by attempting to hijack the topic. Eg.
Someone tries to divert a discussion about an input bug in Devuan to
an argument about the merits of two browsers.

Regards,           ... P.

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work: https://en.wikibooks.org/wiki/User:PeterEasthope