Hello Deng Boping,
Are you referring to the i.MX6D/Q Silicon part numbers? If that’s the case the revisión letters C, D, E refer to silicon revisions 1.2, 1.3 and 1.6 respectively as shown in Figure 1 of the i.MX6D/Q datasheet (link below).
https://www.nxp.com/docs/en/data-sheet/IMX6DQCEC.pdf
Please refer to Engineering Bulletin 803 (link below) for details on the differences between silicon revisions 1.2 and 1.3.
https://www.nxp.com/docs/en/engineering-bulletin/EB803.pdf
Revision 1.6 is very similar to the 1.3 revision (there were no errata corrections) but there were some changes related to the ROM code. You can look at the following link for details on this.
https://community.nxp.com/docs/DOC-334996
I hope this helps!
Regards,