When someone says "use your best judgment" they're not being specific about what they want. It's a worrying phrase to me because when someone uses it, they're often trying to warn me to meet some standard of behavior or dress, but they're being unclear. "Use your best judgment" usually means "read my mind and use my best judgment." It assumes we all think the same way and share the same basic cultural behaviors and beliefs. People who use the phrase "use your best judgment" assume everyone thinks as they do on the topic being discussed.
The assumption that everyone has the same basic knowledge leaves out those of us who get "common sense" wrong all the time. We're the ones who don't know what appropriate dress means in all contexts.We're the ones who put our foot in it because we honestly don't know what questions are okay to ask. We take risks not because we're brave, but because we think bringing up such-and-such a topic in such-and-such a context is perfectly okay. It feels right to us. Too often, we realize our mistake when the damage is already done.
So when someone gives me guidance that includes the phrase "use your best judgment" I stop and ask them to be clear enough for an idiot (me) to understand. Chances are my judgment isn't the same as their judgment. Chances are the lessons I've learned about life and the responses I think are appropriate might not be what they have in mind. If someone won't or can't explain what they mean by "use your best judgment," I'm left to draw my own conclusions and that just doesn’t always turn out well.