The Big Picture of System Messages

All error, warning and informational messages that are displayed in the system are maintained on the message table. Every message is identified by a combination of two fields:

Message category number. Think of a message category as a library of messages related to a given functional area. For example, there is a message category for billing messages and another one for payment messages.
Message number. A unique number identifies each message within a category.

Every message has two components: a brief text message and a long description. On the Main tab, you can only maintain the brief message. If you need to update a message's long description, you must display the message on the Details tab.

Note: You cannot change the base package's text. If the message is "owned" by the base package, you cannot change the base package's message or long description (if you could, your changes would be overwritten during an upgrade). If you want your users to see a different message or long description other than that supplied by the base package, display the message on the Details tab and enter your desired verbiage in the "customer specific" fields (and flush the cache).