Types of 2D electrodes

The description '2D' impies that the geometry is described in terms of only 2 coordinates, which depend on the symmetry of the system.

In planar symmetry the coordinates that we use are x and z. The coordinate x is usually the transverse distance from an axis, while z is the distance along the axis.  The third dimension y is assumed to extend infinitely far in the +/- y directions.  The fields therefore depend on x and z, but not on y.

In cylindrical (or axial) symmetry the system is rotationally symmetric about the z axis.  The only relevant transverse coordinate is the radial distance r from the axis.  The fields therefore depend on r and z, but not on the angle of rotation θ around the axis.

In both symmetries the cross-section of an electrode can be straight or circular or a wide variety of other shapes, as described in the specification details.

Other electrode information is explained elsewhere:

Reflecting electrodes

Applying voltages

Subdividing 2D electrodes