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Have you written some code? Please post it using code tags
@liesS2 please provide a more extensive explanation of the problem.
If you mean that you want the Rx to acknowledge a message that contains 0x12 and not to acknowledge a message that contains 0x85 then that is just not possible.
The Auto Acknowledgement only acknowledges that the message has been received. It does not examine the content of the message. Examining the content is a matter for your Arduino program AFTER the message has been received and acknowledged.
If you explain properly what your project is and how you want it to behave I'm sure we will be able to suggest a solution.