I would like to know the meaning represented by each dimension of the contact map, distance map and HB map.

[Liuyu_20000419]

I read the manuscript "Deep-learning-based single-domain and multidomain protein structure prediction with D-I-TASSER ", where Fig 1a shows the contact map, the distance map and the hydrogen bond network map, I am confused about the dimensions of these three. According to my understanding, these three graphs can be represented by the matrix of L * L. May I ask what the meaning of "2", "3" and "N" here respectively? Why the last dimension of the distance map and hydrogen bond network map are "N" but the last dimension of the contact graph is "1"? And why does the hydrogen bond network have an extra L dimension?
Looking forward to your reply, thank you!