This is only needed when you convert to Equatorial Coordinates or
Ecliptic Coordinates.
When you do, you may select to correct only the precession or both
precession and nutation. I leave this up to you: if you really know
what you are doing you can use 'Just precession' but the 'Both
precession and nutation' is the default. Leave it on, if you do not
know a good reason to change it.