Messages are single texts, stored in table T100, that you can maintain in Transaction SE91 or by forward navigation in the ABAP Workbench. T100 has the following structure:

• Language key
• Twenty-character message class
• Message number
• Message text (up to 72 characters)

Message classes assign messages to an application (of a specific package, for example), and the message numbers identify the individual messages within the message class. When you call a message in ABAP, you need to specify the message class and message number. The language key is supplied by the system according to the system language environment.