Many thanks ptaylor~ Nicely explained.

I've read this mechanism in the help for 3dNwarpApply, but I didn't realize that it also applies to cat_matvec.

An important question arises though: when to use -I to take the inversion?

If I align_epi_anat.py dset1 to dset2, which gives me a dset1_mat.aff12.1D,
and align_epi_anat.py dset2 to dset3, which gives me a dset2_mat.aff12.1D.
Now I want dset1 to align with dset3, shall I do
cat_matvec -ONELINE \
dset2_mat.aff12.1D \
dset1_mat.aff12.1D > combined.1D
Or
cat_matvec -ONELINE \
dset2_mat.aff12.1D -I \
dset1_mat.aff12.1D -I > combined.1D
And then
3dAllineate -input dset1+orig -1Dmatrix_apply combined.1D

In the example, why
FT_anat_ns+tlrc::WARP_DATA -I \ # 1) into TLRC from anat
FT_anat_al_junk_mat.aff12.1D -I \ # 2) into anat from one EPI
are followed with -I, but
mat.r$run.vr.aff12.1D
is not?

Thanks!