The difference is that POR resets the entire SoC, whereas during the COLD reset the Secure JTAG Controller (SJC), Real Time Clock (SRTC) and ARM debug state machine are not reset (remain their pre-reset state). For more information, please refer to the Table 60-4 of the i.MX6Dual/Quad Reference Manual document, available on the processor's Documentation web page:
Have a great day,
Artur
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