If in a horoscope, all seven planets are placed within the axis of Rahu and Ketu; Kaal Sarp Yog is formed in such horoscope; provided both Rahu and Ketu are functionally malefic in the horoscope. It means that even if one of them is benefic in such horoscope, Kaal Sarp Yog may not form in such horoscope.
Based on placements of Rahu and Ketu in various houses of horoscope, 12 types of Kaal Sarp Yog exist; from Anant Kaal Sarp Yog to Sheshnaag Kaal