Procedure to prime the trap, what is the black nipple just below the flue discharge for?

How to prime a Goodman Furnace Trap: https://informinc.org/goodman-furnace/how-to-prime-condensate-trap/

The extra nipples are for application when an upflow furnace is used horizontally. See pages 20-24 for exact details: https://hvacdirect.com/media/hvac/pdf/gmes9-gces9.pdf

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