Max # of AR Iter

What is the function of Max # of AR Iter, and why I couldn’t configure it?

Check out http://www.rtklib.com/prog/manual_2.4.2.pdf . It will be available for change in moving base mode.

Set the number of iteration in the measurement
update of the estimation filter. If the baseline length is
very short like 1 m, the iteration may be effective to
handle the nonlinearity of measurement equation.

How to describe “iteration in the measurement update…” and “nonlinearity of measurement equation” mean?
How do we can measure a parcel with “moving base”?