Controls are calculated heuristically. All words found in the control word lists are then removed from the reference sentence. The remaining words, which represent the content, are used as input into the model, along with their POS tags and lemmas.

In this way we encourage models to construct a sentence using content and style independently. 
 Question: How they know what are content words?
 words found in the control word lists are then removed The remaining words, which represent the content