The PES is divided into 16 wedges, 8 on each plug. Each wedge has 2 layers: u and v, with u being closest to the interaction point (IP). Both layers are required so that a position measurement can be made in 2 dimensions. Wedges are further subdivided into eta regions.
The PES is composed of 6400 scintillator strips in total. Strips run parallel to both sides of the wedge. A fraction of the light emitted by the strips are collected by wavelength shifting fiber (WLS) and read out.
The PES is important because it gives an independent measurement for the energy and position of EM objects. It can also help distinguish between hadronic objects.
For a more detailed description, read the Technical Design Report (TDR)
