Hello Drake,
PCM or ET are simples structures (e.g.: resistors, capacitors and simple transistors), that are placed between the dice (usually in the streets).
The purpose of these structures is to allow the fab to measure process parameters (e.g.: sheet resistance of metal layers, gate oxide breakdown, Ids current and more), to ensure the process is within spec during and after manufacturing of the wafer.
Usually, the fab measures these parameters in 5 or 9 locations in the wafer (e.g.: center, left, right, up and down), but in case of an issue the fab can measure in more locations and even produce a wafer map for each parameter.
