The SentimentEvaluator function uses test data to evaluate the precision and recall of the predictions output by the function SentimentExtractor (ML Engine). The precision and recall are affected by the model that SentimentExtractor uses; therefore, if you change the model, you must rerun SentimentEvaluator on the new predictions.
SentimentEvaluator uses files that are preinstalled on ML Engine. For details, see Preinstalled Files That Functions Use.