Generally speaking, all "Fit" functions accept X and Y arrays and calculate the interpolant stored in an additional Y array, optionally rewriting the original one.They are used to calculate an aproximation of actual data.
On the other hand, "Ev" functions accept only the X array in input and calculate the value of the Y array given some criterion: in case of linear evaluation you must pass slope and intercept, with polinomial one you must pass the whole set of coefficients given the degree of the polynomial. Functions output a new Y array calculated from scratch. Evaluation function are used to try guessing some phenomenon based on a theoric model.
