  The Cassiopeia A supernova remnant has a complex structure, manifesting the
multidimensional nature of core-collapse supernova explosions. To further
understand this, we carried out near-infrared multi-object spectroscopy on the
ejecta knots located in the "northeastern (NE) jet" and the "Fe K plume"
regions, which are two distinct features in the outer eastern area of the
remnant. Our study reveals that the knots exhibit varying ratios of [S II] 1.03
$\mu$m, [P II] 1.189 $\mu$m, and [Fe II] 1.257 $\mu$m lines depending on their
locations within the remnant, suggesting regional differences in elemental
composition. Notably, the knots in the NE jet are mostly 'S-rich' with weak or
no [P II] lines, implying that they originated below the explosive Ne burning
layer, consistent with the results of previous studies. We detected no ejecta
knots exhibiting only [Fe II] lines in the NE jet area that are expected in the
jet-driven SN explosion model. Instead, we discovered a dozen 'Fe-rich' knots
in the Fe K plume area. We propose that they are dense knots produced by a
complete Si burning with $\alpha$-rich freezeout in the innermost region of the
progenitor and ejected with the diffuse X-ray emitting Fe ejecta but decoupled
after crossing the reverse shock. In addition to these metal-rich ejecta knots,
several knots emitting only He I 1.083 $\mu$m lines were detected, and their
origin remains unclear. We also detected three extended H emission features of
circumstellar or interstellar origin in this area and discuss its association
with the supernova remnant.
