How are my order numbers created, and where does the three-letter prefix come from?

Each order gets a number made of a prefix, a dash and a running count, for example ABC-0012.

• The prefix is the first three letters of your store name (numbers, spaces and symbols are skipped), taken from Site Title in Settings → Web Settings → General Settings. If no name can be found, the prefix is ORD.
• The count goes up by one with every order in your store and is padded to four digits.

You can search for an order by this number on Orders → Manage Orders. There's no separate setting for the prefix. It comes from your store name.