Dataset classifications, or "topics" are how we group datasets into categories. It's a way of grouping datasets that might come from different places and sources into categories that can be easily searched in the platform.
To classify a dataset, you may create a new topic or select a preexisting one. This action is performed in the "metadata" section of the create/edit dataset popover. Topics are denoted by a hashtag in the user interface, but you do not need to add a hash when creating one. Simply write the name of the topic to search for an existing topic or create a new one. You can classify a dataset with as many topics as you like.