Hashtag: A word, or a phrase with no spaces (hyphens and underscores are allowed), prefixed with the hash/pound/number/octothorpe character (#). A group member applies a hashtag to a message by including the hashtag within or at the end of the message’s subject line. A maximum of five hashtags can be applied per topic. Hashtags have several uses within Groups.io, including controlling certain behaviors of topics and messages.