Thanks Rick,
That's what I figued.

I need help on a related issue: How to label the sub-bricks while combining them.
I used 3dcalc to convert NIFTI file to AFNI and then 3dTcat to combine my separate result files. But here I am having one issue: How can I define the index label to something I want. If I do it without any options, it makes it 0 and 1. if I used relabel , it takes the whole file name. Is there any way I can change it to my desired label name or I should name my files in a way that they end up giving a good index label.

Regards
Priyank