SAP Standard transaction OBMSG gives details about all messages for a particular application area.
You can select a application area and go to Messages section to see all the message numbers under that application area with information whether its a warning message (W), error message (E), information message (I) etc. in column standard.
For example: M8 is the application area for PO related invoices & invoice verification.
If you get any error in invoice you will get a message number there like M8477, M8483 etc.
OBMSG shows the SAP standard setting for these messages whether its error or warning.
If in your company you want to override the SAP standard message type without changing Standard SAP setting you have to get the message number you can do so in OBA5 transaction.
For example: Standard SAP has message M8477 as warning message, but I want to make it error message in my organization. I would follow below steps:
Call OBA5 --> enter appln area (M8) --> New entries --> enter your message number (477) --> Enter E in Online and/or BatchInput as desired --> Save. Now whenever this message is generated it will be shown as a error message and not a warning message.
In a separate column called Standard you will see the corresponding Standard setting too which is in OBMSG transaction for this message.
Hope the information provided is what you were looking for or gives you some idea about the message control.