No System name and transaction code in SM04

If you have more than one application server, use AL08 instead of SM04.

or

It is because they are at the logon screen which has established a connection.

You will notice that the transaction code shown when there is no user name is SESSION_MANAGER.

This shows you which workstations out there have the login screen up but have not yet entered a user name and password.

The transaction column shows the Last executed transaction code.  

Sometimes your users will have multiple sessions open. If they do, to the system, it is the same as multiple logins as it relates to the resources used etc.  

So the user name will show up more than once in AL08.

Under the application server they are logged into, each instance of that user name on that application server represents a session open.  

For instance if you run AL08, you will have your name show up at least twice on the application server you are logged into.  

One will show AL08 and the other will not have a transaction next to it.  

Then you will notice your user name showing up on all other application servers with no transaction. This is because you are using AL08.