It's easy to get overwhelmed with all the different types and diameters of accessory cords. I usually always get 7mm for making my prusiks and cordalettes, but I've often wondered who uses the smaller diameter stuff?
I naturally assume smaller cord is used for friction knots on smaller ropes, but how small is too small? The answer given to this question: What's the minimum diameter cord to use when climbing a rope using a prusik , states that you should use cord for prusiks that is 2mm smaller diameter than your rope, with a minimum of 5mm, but who on earth is ascending 7mm ropes with 5mm prusiks?
Why are there so many different accessory cord diameters, and what are their intended uses in climbing?