90% of customers will probably be decently served by a sails salesman's recommendation on the cloth.
But engineering types like me do like to see the data on the fabric to see if mr. salesman is blowing smoke up my ass or if the new wonder fabric really has better properties. If its better then...
Sailcloth manufacturing is a weird business. They seem to never want to disclose the stretch/strain/bias etc etc data to the layman/woman.
What is so secret about what you are selling that you can't tell me how stretchy it is?