A perfect electric conductor (PEC) has zero tangential electric field at its surface. FDTD OpenMP exposes a reflecting option named PEC, whose material setter removes boundary absorption:

for (size_t k = 0; k < width; k++) {
    if (k > pec_width && k < width - pec_width)
        continue;
    material->fm_sigma_m[k] = 0.0;
    material->fm_sigma_e[k] = 0.0;
}

pec_width chooses where conductivity is cleared; it does not create a metallic slab or clamp both electric endpoints.

Termination comes from the kernel's loop bounds: Ez[0] and the last $H_y$ sample are not updated. They remain zero from initialization during ordinary stepping. The left endpoint therefore fixes an electric sample, while the right fixes a magnetic sample. The two ends are not symmetric PEC walls. When replacing a PML, clear the region containing the losses that should be removed.

Source: fdtd_ez_material.c.