The extended program check can be called for activated programs either from ABAP Workbench or by using transaction SLIN. It performs static checks that are too complex for the normal syntax check.
Start Extended Program Check from SE38 ABAP Editor:
The errors, warnings, and messages displayed by the extended program check are just as important as the syntax errors and syntax warnings from the syntax check. For example, an error reported by the "Extended Program Check" can indicate that a program definitely produces a runtime error when it is executed. Warnings and messages usually indicate a questionable use of language elements, which is likely to cause unexpected program behavior.