X_full (first paramter): A column vector of floating point data
Y_full (second parameter): A column vector of floating point data.
cs_s (third parameter): A scalar floating point number.
X_full and Y_full are intended to be data coordinates. cs_s is some kind of reduction factor.
According to the authors, datared() is a data reduction function. If you search their publication you can see it being called from other functions.