This section specifies behaviors in which there is optional behavior in one of the standards on which the LSB relies, and where the LSB requires a specific behavior. 
LSB conforming implementations shall support the following options defined within the ISO/IEC 9945:2003 Portable Operating System(POSIX)and The Single UNIX® Specification(SUS) V3:
function shall consume a file descriptor in the same
open, and therefore may fail with
The START and STOP termios characters shall be changeable, as described as optional behavior in the "General Terminal Interface" section of the ISO/IEC 9945:2003 Portable Operating System(POSIX)and The Single UNIX® Specification(SUS) V3.
function shall fail with errno
EINVAL if the
amode argument contains bits other than
those set by the bitwise inclusive OR of
link() function shall require access
to the existing file in
order to succeed, as described as optional behavior in the
ISO/IEC 9945:2003 Portable Operating System(POSIX)and The Single UNIX® Specification(SUS) V3.
LSB conforming systems shall enforce certain special additional restrictions above and beyond those required by ISO/IEC 9945:2003 Portable Operating System(POSIX)and The Single UNIX® Specification(SUS) V3. 
The fcntl() function shall treat the "cmd" value -1 as invalid.
The "whence" value -1 shall be an invalid value for the lseek(), fseek() and fcntl() functions.
The value "-5" shall be an invalid signal number.
If the sigaddset() or sigdelset() functions are passed an invalid signal number, they shall return with EINVAL. Implementations are only required to enforce this requirement for signal numbers which are specified to be invalid by this specification (such as the -5 mentioned above).
The mode value "-1" to the access() function shall be treated as invalid.
A value of -1 shall be an invalid "_PC_..." value for pathconf().
A value of -1 shall be an invalid "_SC..." value for sysconf().
The nl_item value "-1" shall be invalid for nl_langinfo.
The value -1 shall be an invalid "_CS_..." value for confstr().
The value "z" shall be an invalid mode argument to popen().
The LSB does not require the kernel to be Linux; the set of mandated options reflects current existing practice, but may be modified in future releases.
These additional restrictions are required in order to support the testing and certification programs associated with the LSB. In each case, these are values that defined macros must not have; conforming applications that use these values shall trigger a failure in the interface that is otherwise described as a "may fail".