What is the "Feature" meaning in gmx_clusterByFeature?

Brief description of tools/files:
I have performed “gmx_clusterByFeature” (Welcome to gmx_clusterByFeatures’ documentation — gmx_clusterByFeatures 0.1.17 documentation), and follwed the Working Examples (PCA-based ligand clustering Clustering ligand conformations using cartesian PCA — gmx_clusterByFeatures 0.1.17 documentation). Than, I got the plots of feature-vs-feature. (like the feagure below)

My question is: How can I know what the meaning of feature1~6 in this work?

Link:
Contact person (name and email address):

How the work has been tested/reviewed:

  1. Download and Installation gmx_clusterByFeatures

  2. echo 13 14 | gmx covar -s input-files/input.tpr -f input-files/trajectory.xtc -n input-files/input.ndx (13 index group of receptor, 14 index group of ligand)

  3. echo 13 14 | gmx anaeig -s input-files/input.tpr -f input-files/trajectory.xtc -n input-files/input.ndx -proj -first 1 -last 20

  4. echo 0 14 13 | gmx_clusterByFeatures cluster -s input-files/input.tpr -f input-files/trajectory.xtc -n input-files/input.ndx
    -feat proj.xvg -method kmeans -nfeature 20 -cmetric ssr-sst -ncluster 15
    -fit2central -sort features -cpdb clustered-trajs/central.pdb
    -fout clustered-trajs/cluster.xtc -plot pca_cluster.png\