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 5.1.13 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.