Sense vector ξ is one of the two vectors used to describe the dislocations in crystals and is a unit vector parallel to the dislocation line at a particular point. The other vector of the two is Burgers vector b.
Note that when the sense vector is reversed, the resulting Burgers vector is also reversed. Therefore, one of the characteristics of the dislocations is represented by the cross product of the sense vector and the Burgers vector, always pointing to the extra half plane of atoms that defines the edge dislocation: extra half plane, regardless of which direction the sense vector points as shown in page3459.
Furthermore, a glide dislocation is generally a line boundary between the slipped and unslipped portions of the glide (slip) plane. The line is not necessary to be straight. When it is curved some parts can be a screw dislocation and some parts an edge dislocation. For a given sense vector that points in one direction along the dislocation, the Burgers vector is invariant.