This parameter controls whether the piecewise-linear approximation of a function constraint is an underestimate of the
function, an overestimate, or somewhere in between. A value of $0.0$ will always underestimate, while a value of $1.0$
will always overestimate. A value in between will interpolate between the underestimate and the overestimate. A special
value of -1 chooses points that are on the original function.

See the discussion of function constraints for more information.