Function offset_shell_by_boolean 

pub fn offset_shell_by_boolean(
    source: &BrepSolid,
    opening_face_ids: &[u64],
    distance: f64,
) -> Result<OffsetShellResultRecord, KernelRefusal>

The textbook shell: the offset body with the opening faces HELD (copied at distance zero, so they cancel in the difference), then source − body for an inward shell and body − source for an outward one. The walls at the openings come out of the boolean. distance carries offset_shell’s sign (positive inward).

This is the lane offset_shell falls back to when its direct construction refuses by one of its own names ([direct_refusal_takes_the_boolean_lane]); it is public so the two can be compared on any fixture. The result’s face_images are read back from the operands by surface ([provenance_by_surface]): Offset for a face of the body’s skins, Wall for a held opening face, Source for a face of the source.