Plumbing question: Y connector on vertical drain

[Dave]

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?

[dpb]

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?