A perfect electric conductor (PEC) forces the tangential electric field to zero at its surface. In the project's TMz formulation, $E_z$ is tangential to every wall of the $xy$ domain, so its boundary value is:

$$ E_z\big|_{\mathrm{wall}}=0 $$

FDTD CUDA enforces this on the first and last rows and columns. The boundary update clears $E_z$ and its split components $E_{zx}$ and $E_{zy}$ at those cells. The magnetic field continues to advance from differences in the electric field.

PEC reflects waves back into the domain. This makes it useful for a closed conducting box, but a reflected pulse can later cross a probe and overlap the original signal. An absorbing boundary is preferable when the intention is to approximate an open domain.

Select Boundary::Pec in the API or --boundary pec in the driver. It is also the default condition. The simulation rejects $E_z$ sources on the PEC edge because those samples are held at zero.