SE91 is used for Message Maintenance and Report used to generate SE91 is: RSMESSAGES
Messages allow you to communicate with users from your programs. They are mainly used when the user has made an invalid entry on a screen.
To send messages from a program, you must link it to a
message class. Each message class has an ID, and usually contains a whole
set of message. Each message has a single line of text, and may contain
placeholders for variables.
Stored Data
All messages are stored in table T100. You create and
edit them using Transaction SE91. Once
you have created a message, you can use it in the MESSAGE statement in
a program.
Starting the Message Maintenance Transaction
1. Using forward navigation from the ABAP Editor.
2. You can display the messages for your program from the ABAP Editor by choosing Goto -> Messages. The Maintain Messages screen appears. By default, the system display the message class linked to the current program.
3. You can also enter Transaction SE91.
If you choose Goto -> Messages from the ABAP Editor and
your program does not have a defined message class, the system assumes
you want to browse an existing class and prompts you for a message class
ID.
Error Checking
SE91 message can help you to trace your error messages by making used of the Where-Used List features.
Personally, I used it quite often when there is an error message coming out from the SAP transaction code.
