Thank you Dr. Cox for your explanation.
My problem goes like this.
I have an AFNI anatomical dataset which was transformed into the ANALYZE format (3dAFNItoANALYZE) for segmentation purpose (using SPM). After the segmentation, I wana transform the result back into AFNI format (3dcopy). However, I found the original and transformed-back datasets
do not match with the A-P mirror flipped. Given the header informations exactly match between the two datasets (ascertained by comparing the 3dinfo), I just want to A-P flip one of the data body. Here I just need a command "3dAPflip".