So in this mode the IV is replaced by non random but unique nonce for example the numbers 1,2,3,4,5, could all be used as a nonce, and now, the appeal of this mode is that if the recipient actually knows what the nonce is supposed to be then there's no reason to include the nonce in the ciphertext, in which case, the ciphertext is exactly the same length as the plaintext, unlike CBC with the random IV, where we had to expand the ciphertext to include the IV, here, if the nonce is already known to the recipient, there's no reason to include it in the ciphertext, and the ciphertext is exactly the same length as the plaintext. 