The messaging substitution codes are listed by category of message they are used in. This doesn't necessarily mean that they are limited to that particular message type - a lot can be used in message types other than the types they're listed in.
The messaging system understands the context in which the codes are being used - both by the Message Type and the section that the code is in. This means that the same code, used in different parts of a message will produce different results. An example is pricing codes. Using the code to output the agent price in either the message header or tail will output the booking agent price. Using the same code in the <<services>> section will output the service price.