POR = POssession Rush 214, 66
PORC = PO Rush Cancel 214, 66, 4
POS = PO Sidestep 214, 2 or 8
POSC = PO Sidestep 214, 2 or 8, 4
POR and POS as well as Hover (PO 236), Silent Mist (PO 214) and PO B+K are mainly for evasion. They are needed because even on hit slow PO transitions like AB PO and slower allow the opponent to beat Taki out of any regular PO move (except PO 8K, which is unsafe even on hit) with a fast TC(tech crouch) attack like Taki's own 2A or maybe 3bK.
PORC and POSC are needed if you evaded and wanna use a non PO move like a throw.
Or with Guard, when the opponents are fed up with your evasion and start using lockdown moves like Asty's 66K (TC, hits even POS when timed well) or Sophi's 236AA (also TC, also lockdown)
You use PORC G and punish said lockdown moves because those are mostly more or less unsafe.
@ Sig: 33 or 99A PO is her fastest PO transition and here nearly everything is useful PO B for example is uninterruptable in this setup. You can also just use 33_99[A]BBB for the delayed mid stabs.
@ Kikoshoo: The low kick to mid kick you mean is the above mentioned FC(full crouch) 2A+K, K.
It's a natural counter combo (NCC) and when only the second kick hits as a counter it gives unshakable crumple stun for nice combos.
It's also safe on block.