This command is in the Matrix menu. It allows to fill in a matrix with the results of a function z=f(i,j) in which i and j are the row and column numbers.
You can use the X-values and Y-values defined with the Set Dimensions... command command, and also define your functions based on the x and y variables.
The functions can be written on several lines, and the intrinsic functions which are available are listed in the appendix .