The Merge button helps optimize meshes when importing 3D objects. It welds points which overlap in the mesh. Import options are available only when the Import button is pressed, or the PolyMesh3D tool is selected.
The Add button specifies that the next import action adds the imported mesh to the current one. If unpressed, the current mesh will be replaced.
The Triangles to Quads slider affects imported meshes. If the imported mesh is composed of triangular polygons, ZBrush can combine some of the adjoining triangles to four-sided polygons. If two adjacent triangles form an angle less than the value of this slider (in degrees), they will be combined into one four-sided polygon. If this slider is 0, this function is disabled.
The Weld Points slider affects imported meshes. Adjacent points separated by a distance less than the value of this slider are welded into a single point. If this slider is 0, this function is disabled.
The Import Tool button imports a 3D object. Additional import options are available in the Tool Import menu when the PolyMesh3D (or another imported tool) is selected.