The account number is a numeric field, so it can't contain letters or other characters. Those two digits represent the department. They track department costs, such as rent, supplies, and salaries. They are different than profit centers. Profit centers are used to track performance of different business units or divisions within the agency.
Departments are optional.
Each account has an optional 2-digit department suffix, which limits you to 99 departments. And an account can't begin with a zero and can't contain letters, dashes, or other characters. An account can't be deleted if it has activity, even if the balance is zero. Once these JEs are purged by the year-end closing procedure, the account can finally be deleted. The account number doesn't affect subtotaling on financial statements.