User Exits: Global Variables
When you work with global variables, you need to specify a replacement type to determine how the variable should be replace with a value. In most cases, the replacement types provided by the system (automatic, manual, SAP exit, fixed value from table) will be sufficient.
If you need to have a variable replace using a different method, you can program a user exit for global variables to define your own replacement type.
You can read about the user exit in Customizing under Tools ® SAP enhancements (development class KC, enhancement KKDR0001). There you will find the technical information necessary for programming an exit as well as a code example.