2017 Polyline controls Viewport – But how?

Hi all,
I am not sure how this happened, but I like the result of having
a viewport that clips when I move the polyline somehow associated
with it.

OK…I understand I can place a viewport on a layout, and then
draw a polyline to use as a clipping boundary. But, how is it
that I ended up with one viewport that actually is being
controlled by the polyline as I drag it around? I have a hunch
it is tied to my activity of copy/clipping from one file to
another, but I don’t know. Regardless, I like the affect. As it
is, if I hover my cursor over and select the polyline, and look
at the quick properties dialog box, I can see that two (2)
objects are selected – a polyline, and a viewport. If I select
either one, and erase it, both disappear. Can anyone tell me
more about this behavior, and how to replicate it, please?

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