About the unit

The default unit for a box is 1,but the unit of 1 for shell thickness is different from it, what is the ratio or relationship between them?

It’s a relative unit (technically, 10% of the local bounding box radius of your mesh).

I might change that later (but absolute values needs to be handled with care as most people have no idea what is the real size of the model).

Ideally I’d like to rethink the mask thing to be more intuitive, with visual feedback.