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Thank you for flagging! We've come across them in prior searches, but interesting to learn how well-known they are

It's also a major component of the 3d software Houdini, and is used for physics simulations around cloth, grains, hair, etc. https://www.sidefx.com/docs/houdini/vellum/index.html

Yup I came here to say this too. It's a well-known product in the self-publishing world.

Likewise, it is a well-known app among writers.