This function returns the index (greater than or equal to 0 (zero)) of the unbound constrained variable with the smallest domain from among the constrained variables in the array of constrained variables, vars. If all those variables have been bound already, then this function returns -1.

This function is a predefined choice function, one of the criteria that you can use to set parameters for the order in which Solver binds constrained variables during the search for a solution.