What is the role of the [UNK] token in vocabulary management?

The [UNK] token, short for 'unknown' token, serves as a placeholder for words that are not present in the model's vocabulary. Vocabulary management is a crucial part of preparing text for a language model like ChatGPT. The vocabulary contains all the words the model recognizes. When the model encounters a word it hasn't seen during training, it cannot directly process that word. The [UNK] token provides a way to represent these out-of-vocabulary (OOV) words. Instead of ignoring the unknown word entirely, it is replaced with the [UNK] token. This allows the model to still process the sentence without crashing and to potentially infer something about the context of the unknown word based on the surrounding words. The [UNK] token signals to the model that it is dealing with an unfamiliar word, and it can then use its knowledge of language structure and context to make informed predictions, even without knowing the specific meaning of the replaced word. For example, if the word 'flibbertigibbet' is not in the vocabulary, it would be replaced by [UNK], and the model would process the sentence containing [UNK] using the context of the other words.