Additional Data in AEAD (Chacha20-poly1305 libsodium)
First of all the Additional Data (AD) is not a tag. It is data that is also authenticated by the authentication tag. This authentication tag is appended to the ciphertext by libsodium. The tag doesn't consist of separate portions for AD and the ciphertext (and IV), the AD is taken into account during calculation of the tag.

The AD can be any data, including data that can be generated or data that is known in advance. In that sense it does not have to be send. How it is handled or generated is out of scope for the definition of the authenticated cipher. You do need all the AD data on both sides or you won't be able to calculate the authentication tag. The AD cannot be extracted from the ciphertext nor from the tag.

To my knowledge all authenticated ciphers already include the IV in the tag calculation, so there is no need to treat the IV as AD. It's actually a requirement for the CEASAR competition, so no doubt this algorithm will comply with that. Don't confuse or mix nonce and AD, they are different inputs to the cipher.

If you want a standard way of handling nonce, AD etc. you should look for a suitable container format such as CMS.