Noting digressions in a conversation
That’s beside the point.
That’s beside the question.
That’s not at issue.
That’s not the issue.
That has nothing to do with it.
That has nothing to do with what I’m talking about.
That’s another story.
That’s a whole ‘nother story. (folksy)
That’s a different ball of wax. (idiomatic)
ball of wax = thing; matter
That’s a different kettle of fish.(idiomatic)
kettle of fish = thing; matter
That’s another can of worms. (idiomatic)
can of worms = set of problems
That’s a horse of a different color. (idiomatic)
a horse of a different color = a different kind of problem altogether
You’re off on a tangent.
You’re getting off the subject.
As you were saying…
Getting back to the point…
But I digress. (formal)