What happened to unknown words?

[zhangriqi]

Hello, I noticed there's in the dictionary and we only keep the most frequent words in the dictionary. But I don't really understand what happened to the new words (they are all 'unk' in the dictionary, is that right? )that's only in the test data but not in the training data set? Please tell me what I'm missing. Appreciate it.

[ruidan]

words not in the vocab will be mapped to a special token "<unk>".

[Gwynny]

How is it mapped to embeddings? Is any place for them in embedding matrix? Also for padding.