Qortal's Chain Messaging system, is a system that utilizes blockchain transactions, to send messages that are reliant upon chain confirmations in order to arrive. This means that if you want to send a long-term message that will stay on the chain forever, either to a person privately, posted publicly to be read with a call to the API, or to a group on the chain, this method is for that. However, this messaging method is not free, requires a transaction fee to use, and is reliant upon the block times for confirmation prior to being sent, received, and readable by the other party/parites/API.
Essentially, use this method of messaging as you would for an important email, or for something you would like to be secured in time on the chain forever, like details for a project so you can be known to have done it first, etc.
This system can be tied into a new application that could potentially be used for 'copyright-like' use cases.