The READONLY specifier indicates only READ statements can refer to this connection. It takes the following form:
READONLY is similar to specifying ACTION='READ', but READONLY prevents deletion of the file if it is closed with STATUS='DELETE' in effect.
Default file access privileges are READWRITE, which can cause run-time I/O errors if the file protection does not permit write access.
The READONLY specifier has no effect on the protection specified for a file. Its main purpose is to allow a file to be read simultaneously by two or more programs. For example, use READONLY if you wish to open a file so you can read it, but you also want others to be able to read the same file while you have it open.
For More Information:
For details on file sharing, see your user manual.