Message content is part of the Cognitive architecture.
Messaging is the only way agents communicate to each other. So messaging model represents the interaction mechanism of agents. This is one of the means of ensuring the autonomy of agents. A message can have different types: it can be a phone call, a meeting, an email, … Message content helps you define the content of a message : knowledge, beliefs, referral, …
Each agent may be able to send or not knowledge (or information) and to receive or not knowledge.
You can use a book to learn new knowledge. As an agent, a book is able to send knowledge, but not able to receive knowledge.