Plumbing question: Y connector on vertical drain

I had a plumbing inspection done, one of the comments stated: Y connector not to be used on a vertical drain line.
Can anyone elaborate?
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Dave wrote:

Since nobody else jumped in, I'll comment...
Need more info, I think. I'm unaware of there being a general total prohibition against it, but I'm not a plumber. Would have to know the arrangement to try to understand the possible problem/concern. Could a sweep L be used instead and satisfy him?
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