Zan2.0s are BP models of the AP mailbot, the Zan.
The reason that the Zan is an AP and the 2.0 model is a BP is because of the large memory difference between the two. The 2.0 has much more memory and is able to carry large files and bits of data to high priority programs. The Zan 2.0 is even able to carry AVFs, to eliminate large hordes of viruses.
As many important things can be carried out by Zan2.0s, Spartan 5s usually escort the mailbots.