Under the settings for your classifier, you can use the Tagging strategy to define if you prefer your model to assume that each text send to classify should belong to one and only tag, a set of tags, or if you prefer MonkeyLearn to detect it for you:
Single-tag: The model will always tag text with just one tag.
Multi-tag: The model will try to tag a text from a set of tags.
Autodetect: MonkeyLearn will detect your preference automatically based on the data you used to train the model.