Hashtags are words that start with '#'. In the case of Groups.io, they only apply to the subject of a message. So, if you send a message to your group with the subject 'This is a sample message #test', the message will be categorized with the hashtag '#test'.

You can search the archives by hashtags. And you can set hashtags so that any messages tagged with them will be deleted automatically after a specified period of time.

Muting Hashtags

An advantage of tagging messages is that members can mute hashtags that they're not interested in. If someone is not interested in the #FT4 tag   they don't ever have to see any of those messages.