The USEROPEN keyword specifies a user-written external function that controls the opening of the file. It takes the following form:
USEROPEN = function-name
The function name must be declared EXTERNAL, and if it has a data type, it must be INTEGER*4.
With USEROPEN, knowledgeable users can employ additional features of the operating system that are not directly available from Fortran, while retaining the convenience of writing programs in Fortran.
For more details on USEROPEN, see your user manual.