Hello Gavin Liu,
The difference between these two part numbers is just the packaging. Part number MCIMX6S5EVM10AC comes in trays, par number MCIMX6S5EVM10ACR comes in tape and reel packaging.
In the part details page you will see that these are handled as two buying options depending on what package you would require:
https://www.nxp.com/part/MCIMX6S5EVM10AC
I hope this helps!
Regards,
