_INCLUDE System Variable
Specifies a default header file included with user-defined classes, forms, or form sets.
_INCLUDE = HeaderFileName
Specifies a default header file of predefined compile-time constants automatically included with user-defined classes, forms, or form sets. If your header file is in a directory other than the current default directory, include a path with the header file name. Contains the empty string by default.
You can also specify the default header file with the Default Include File item in the File Locations tab of the Options dialog box.