A routing number is a unique identifier that determines where your account was opened. In fact, the routing number comes as a great use whenever you have to transfer money online. A routing number is also known as RTN or an ABA routing number. Ideally, it is a nine-digits code that determines the location of the bank in the USA. If you have a valid bank account in any certified American bank, you can log in to online banking and find your RTN. As far as RTN on Cash App is concerned, you don't need to log in to your bank's website because in just a few simple clicks you can find your routing number on Cash App .