The shrinkwrap displays the union of all object faces cut by the section line. The Outer Shrinkwrap component represents the outline of solid, filled components, such as a wall shrinkwrap. The Inner Shrinkwrap component represents the outline of holes which have been cut, such as a space. These components are specific to 2D sections.
You can exclude individual objects from contributing to the shrinkwrap. For example, you might do this for glass components in windows.
2D section with red outer shrinkwrap
Window glass excluded from shrinkwrap
The Outer and Inner Shrinkwrap components cannot be defined by a material component.