This reply helps a lot. My query is (which might be silly), since you said:

"""
- Such a cluster table is produced by 3dttest++ -Clustsim,
though it is up to you to apply the threshold/cluster step
(it does not assume which p-values to apply).

- The thresholding and clustering is applied to a t-stat or
z-score volume. The mean volume would probably be used for
colorization, showing the effect size.
"""

I got multiple tables after running 3dttest++ -Clustsim on 20*2 subjects, how do you proceed from there i.e. how do I apply thresholding and clustering now on z-score maps for each subject? I don't know how to interpret the results. If I open AFNI, select some p-value and cluster-size value, I get a Report which says peak-x, peak-y, peak-z values which again I don't how to interpret. Any help or direction is appreciated.