It comes purely from the performance manual. Factors involve take-off weight, runway length, runway condition and prevailing weather. The weather, amongst other things would determine whether or not you'd need eng anti-ice therefore incurring a perfromance penalty.
Must say, the 737 using flap 1 for departure doesn't happen very often. It's usually airline and airfield specific and I believe the 400 can't do flap 1 take offs. Flap 5 is the norm.
Hope that helps
