A FF is a generic term,D97 is a low stretch hmpe string that is considered a FF type string.Any string other then B50,B55 0r B500 is consider a FF type string.There are so many different FF or low stretch strings to choose,D97,8125,450+,Ultra cam and many more.A flemish string is 2 to 3 seperate bundles of string material twisted together.You can use different colors,looks like a barber pole.Any bow made for the new low stretch materials you can use B50 or any newer modern material.