-rf argument when applying the warps/affine

[ShuonanChen]

what is the purpose of -rf argument here in the quick start?

greedy -d 3 \
    -rf fixed.nii.gz \
    -rm moving.nii.gz resliced.nii.gz \
    -ri LABEL 0.2vox \
    -rm moving_seg.nii.gz resliced_seg.nii.gz \
    -r warp.nii.gz affine.mat 

i understand that in the documentation of-rf it says "Images will be re-sliced into this space.". But haven't warp.nii.gz and affine.mat learned where in the space to transform already? So i dont understand why we need to specify the fixed image here..

thank you!