Extra Vocabulary contents

huanying, ke, danyuan ... for example. Why are they given as "Extra Vocabulary" in so many chapters?

I have made a note about this numerous times in Rocket Chinese. There are many potential words and phrases that could be included but are not. A lot of the lessons have sentences that add lots of new vocabulary and grammatical constructs: these would be good candidates for extra vocabulary. This matters because it affects what appears in the testing tab. If some of the new individual words and phrases were included in the extra vocabulary, they would appear in the testing. That would help when drilling "Know It" or "Say It". Also, consider adding the words and phrases that are given in Rocket Review.

I think most of them important as far as i get to. but well i guess u can skip any of u think u might not need them.. in my view extra better than less