The FILEOPT=fopt clause is optional. fopt is a character expression. Possible values are 'NOPAD', 'BUFFER=n', and 'EOF'.
'NOPAD'--Do not extend records with blanks if you read past the end-of-record (formatted input only). That is, a short record causes an abort with an error message, rather than just filling with trailing blanks and continuing.
'BUFFER=n'-- This suboption is for disks. For tapes, we recommend the TOPEN() routines because they are more reliable. It sets the size in bytes of the I/O buffer to use. For writes, larger buffers yield faster I/O. For good performance, make the buffer a multiple of the largest record size. This size can be larger than the actual physical memory, and probably the best performance is obtained by making the record size equal to the entire file size. Larger buffer sizes can cause extra paging.
'EOF'--Opens a file at end-of-file rather than at the beginning (useful for appending data to file), for example, FILEOPT='EOF'. Unlike ACCESS='APPEND', in this case, both READ and BACKSPACE are allowed.