What is the 3DS swapnote code?

The 3DS Swapnote code, also known as the friend code for Swapnote (or Nintendo Letter Box), is a unique identifier assigned to each Nintendo 3DS user that allows them to send and receive messages and drawings with friends. To share Swapnote messages, you need to exchange these friend codes with others. However, please note that the Swapnote service was discontinued in 2013, so it is no longer possible to use this feature on the Nintendo 3DS.