Hallo,
einige Zeilen höher im Datenblatt ist auch die Funktion der CKOPT Fuse
beschrieben:

"The CKOPT Fuse selects between two different Oscillator amplifier modes.
When CKOPT is programmed, the Oscillator output will oscillate a full
rail-to-rail swing on the output. This mode is suitable when operating
in a very noisy environment or when the output from XTAL2 drives
a second clock buffer. This mode has a wide frequency range.
When CKOPT is unprogrammed, the Oscillator has a smaller output swing.
This reduces power consumption considerably. This mode has a limited
frequency range and it cannot be used to drive other clock buffers."

Gruß Jan