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This is only a guess, as I have not actually looked at the drains in question.
It could be that "A" is some sort of compressed air delivery nozzle and that "B" is supposed to represent the pulling of a hose off a fitting, the fitting being that taped business.
I can't imagine that you are supposed to squirt any sort of fluid in there, as you are likely to end up having a bigger mess than you started with.
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