I used to think I knew the definition of a bl/yf relationship, but opinions vary so much nowadays that I don't bother trying to guess anymore. When a BL mentions a yf to me these days, I just ask them "what definition of yf are you using?" and let them explain what they mean.

Opinions vary greatly. Here are some I've heard:

1. a boy you are in a sexual relationship with.
2. a boy you share a very close relationship with, who is aware that you are a BL and what that means.
3. a boy you share a very close relationship with.
4. a boy you are friends with.
5. a boy you are in love with.
6. a boy you like.

Personally, I only use the word yf (or young friend) to describe boys who fall into categories 1, 2, or 3 above. But, like I said, I have heard people use it to describe boys in all of those categories.

So, I think ultimately, it's up to you to choose what it means when you use it.