In Nominal Editor, a Message is the main way to communicate between components. Each message contains different types of data, allowing components to share information between each other.
Component's may need data from another component to do a specific task, thus needing a Input Message. These messages have the prefix "In_", meaning the message has been created by another component.
Component's might want to write some output data, which in most cases there is at least 1 Output Message. These messages have the prefix "Out_", meaning the message was created by the current component.