It is a compiler directive.
You can like oracle error code (ora - xxxxxx) to a user defined exception.
When the ora eror occurs, the control goes to the exception (if handled )
pragma exception_init (exp, -00001);
when exp then
the control will go the the exception part and handle the user defined exp.
As the error code -1 is linked to exp, no need to write
I think this explains what you want.