So I've actually built patterns using their system. Basically you define a layout using JS and defining a series of points and offsets and lines. You can refer to variables such as body measurements or other dimensions for bags. https://freesewing.eu/ is their more "consumer" facing site where you can enter your measurements and then download sewing patterns sized for you specifically.

One of the other nice things they do as part of the pattern design process is testing the pattern makes sense at many "scales" and so you can actually define a "body" the size of a doll and use this for defining dolls clothing, or make really size inclusive clothing, there are members of the community with varying disabilities such as forms of dwarfism who otherwise struggle to find appropriately sized clothes.