What are Session groups and communities?

Session groups, communities:

A) GROUP: create under + button, "Create Group". Inside group, click image and then corner button to add additional contacts.

B) COMMUNITY: join existing https://sessioncommunities.online/ (copy community address on that page and paste under Session + button, Join Community)

C) COMMUNITY: create new https://internetlifeforum.com/showthread.php?22814-Session-messenger-how-to-create-new-open-community-group

What is the difference between communities and groups?

Open groups (now known as communities) are groups where each member is technically speaking in a public-facing and visible chat window where anyone can also convene, observe, and participate in the chat, as if it were an open and public space. Closed groups do not operate this way; they are invitation-based only, and as opposed to Session's open communities, they can only host up to 100 people. Session open groups (communities) have no known space limit to their entry or number of participants (group members).