What a tier is
A tier is the level of membership someone holds. A member has one.
It is not what they pay. Price lives on a dues schedule, which points at a tier. Keeping the two apart means you can change next year's price without touching anybody's membership, and you can run a different price for the same tier in a different year without rewriting history.
Creating one
Settings, then tiers. Give it a name. That is the required part.
Each tier also gets a short URL-safe slug, derived from the name if you do not supply one. The slug is what the API and the join widget use, so it has to be unique within your organization. If you create a tier called Gold and later create another called Gold, Sembr will tell you the slug is taken rather than accepting it and failing later.
Ordering
Tiers have a display order. It controls the sequence they appear in on the join form, the member record, and anywhere a member picks one. Put them in the order you would say them out loud.
Turning one off
Deactivate rather than delete. A deactivated tier stops being offered to new members and stays on the records of everyone who holds it.
Deleting a tier that members hold would leave those members pointing at nothing, and the history of what they bought would be gone. Deactivating keeps the record true.
What tiers are not for
Use a category for grouping people by committee, chapter, region or interest. A member can hold many categories and they cost nothing. See members.
Use a tier only for the thing they bought.
Changing somebody's tier
Edit the member. Their next renewal prices at the new tier's schedule. Nothing is retroactively adjusted, and no difference is charged or refunded automatically. If money should move because of the change, raise an invoice or a refund for it so the reason is visible on the record.