[docs] clarify FDFD-to-FDTD field reconstruction

README.md

@@ -194,6 +194,10 @@ For a broadband or continuous-wave FDTD run:
    part of the simulation, and compare the extracted phasor to the FDFD field or
    residual.
 
+This is the primary FDTD/FDFD equivalence workflow. The phasor extraction step
+filters the time-domain run down to the guided `+\omega` content that FDFD
+solves for directly, so it is the cleanest apples-to-apples comparison.
+
 ### Real-field reconstruction workflow
 
 For a continuous-wave real-valued FDTD run:
@@ -211,4 +215,14 @@ For a continuous-wave real-valued FDTD run:
    pieces to see whether the remaining mismatch is actually in the mode or in
    weak nonguided tails.
 
+This is a stricter diagnostic, not the primary equivalence benchmark. A raw
+monitor slice contains both the guided field and the remaining orthogonal
+content on that plane,
+
+$$ E_{\text{monitor}} = E_{\text{guided}} + E_{\text{residual}} , $$
+
+so its full-plane instantaneous error is naturally noisier than the extracted
+phasor comparison even when the underlying guided `+\omega` content matches
+well.
+
 `examples/waveguide_real.py` is the reference implementation of this workflow.