About the Attach Mode
When you attach a Freestyle shape or mesh elements, the corresponding vertices are attached or constrained to the hybrid body. The attached vertices are highlighted in the graphics window. When you drag these vertices, they move along the geometric boundaries of the hybrid body.
You can apply a resolution to the attached shape. The selection intent is remains intact when you apply the resolution. New vertices with a higher resolution level remain selected when you select the parent element. If a parent mesh element is attached, the children elements are attached to the hybrid body.
A new shape created in the Attach mode does not automatically attach to the hybrid geometry. You can select such a shape or its elements and manually attach them. When the Attach mode is turned off, you can drag the vertices beyond the geometric constraints of the hybrid body.
Hybrid body refers to any geometry that is not a b-rep (boundary representation) geometry. Facets, triangulated or tessellated models, and volumetrically represented geometries are some of the examples of a hybrid body.