What does it mean to say the chimney inlet must enter from the side? I assume it means side of the flue. How would you verify this in the field?
Good question.
That was taken from the IRC;
R1003.16 Inlet
Inlets to masonry chimneys shall enter from the side. Inlets shall have a thimble of fireclay, rigid refractory material or metal that will prevent the connector from pulling out of the inlet or from extending beyond the wall of the liner.
Open for interpretation on the meaning.
Good question for fastreply@internachi.org
or ben@ education@internachi.org
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As opposed to the bottom of the flue.
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That makes sense, thanks everyone!
As mentioned, you don’t want to enter a masonry chimney any other way other than its side because things fall and can clog up the inlet. Masonry chimneys require a cleanout/buffer space below the inlet – because things fall.
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I haven’t seen any masonry chimneys that could be hooked up from the bottom, but I guess that doesn’t mean they are not out there.
Of course you have! it’s called a Fireplace 
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I believe this was about vents to the side of a masonry chimney. A fireplace chimney is a different animal.
Already been addressed…
(Page 8, Inlet)… https://education.nachi.org/coursemedia/course-153/documents/StudyGuide_%20How%20to%20Inspect%20Fireplaces%20Stoves%20and%20Chimneys.pdf