The eight node plane strain element is a general purpose plane strain element. It is actually a special case of plane stress element: the treatise of Section 6.2.22 also applies here. In addition we have:
Plane strain elements are used to model a slice of a very long structure, e.g. of a dam.
If a plane strain element is connected to a structure consisting of 3D elements the motion of this structure in the out-of-plane direction (z-direction) is not restricted by its connection to the 2D elements. The user has to take care that any rigid body motion of the structure involving the z-direction is taken care of, if appropriate. This particularly applies to any springs connected to plane strain elements.
Notice that structures containing plane strain elements should be defined in the global x-y plane, i.e. z=0 for all nodes.