[R] slice usage

[Nair, Murlidharan T]

I am trying to understand the usage of slice while plotting. Can any one provide me with some explanation to it or point me to a resource where I can read it in greater detail. I have an example from the usage page for SVM.

library(MASS)
library(e1071)
data(iris)
m2 <- svm(Species~., data = iris) 
plot(m2,iris, Petal.Width~Petal.Length)
readline(prompt = "Pause. Press <Enter> to continue...")
plot(m2, iris, Petal.Width ~ Petal.Length, slice = list(Sepal.Width=3, Sepal.Length=4 ))

How do I decide Sepal.Width should be set to 3 and Sepal Length set to 4?

Thanks../Murli