Why do I see a green "EXT" on some of the group conversations?

If a conversation (both group conversation OR direct messages) include at least one user who is not part of your org you will see a green indicator "EXT" next to those topics in the timeline. This indicator means there are members in the conversation who are not part of your organization.

Thread Tips: 

• No "EXT" indicator will be shown if all the members of the conversation are part of your organization.