What is the purpose of a hallmark node for NXT?
From the NXT wiki
A hallmark is a "stamp of approval" for a Nxt node. By creating a digital signature based on your IP address and secret passphrase (private key), you are verifying that your account "owns" a node and is accountable for it. This helps protect the network from attack, and increases the network's trust in your node. You do not need to generate a hallmark if you are a casual Nxt user. It is a very good idea for those who operate public Nxt nodes on dedicated, public servers. Hallmarks have no impact on forging.