ABAP Keyword Documentation →  ABAP Glossary → 

System Interface

Interface of the Class Library integrated into system classes. It is used to implement system functions in separate classes or is used as a tag interface. As a rule, the name begins with IF_ABAP_ or IF_SYSTEM_. Use the input help for IF_ABAP_* or IF_SYSTEM_* in Class Builder to display an overview of all system interfaces.