What fans get is built from items: a release, files, merch, a ticket, or a credit (like a fan's name in the liner notes). A tier is a price plus one or more items.
An item's run size caps it across every tier that includes it, and a tier's limit can only be lower. Price steps raise a tier's price after a number of backers. A fan keeps the price they saw when they saved their card.
Once an Encore takes pledges you can add tiers, add steps and raise limits. A tier with backers can't be removed or lowered, and prices can only go up.