Trouble understanding axis_rule

[gionti]

Hi, I'm having struggles in understanding the axis rules that one can specify in the .settings file of the training. I'm particularly referring to https://github.com/deepmodeling/deepmd-kit/blob/master/examples/water/train/water.json, line 17.

I seem to understand that for each atom, two axes are possible. Each axis is a three-dimensional vector (a_or_r, type, idx), and so it makes sense that there are 2x2x3 values specified.

I don't understand nonetheless the exact settings.
What is meant exactly with axis here? In (a_or_r, type, idx), would (0, 1, 1) mean a and (1, 1, 1) mean b? What is exactly idx?

I thank you in advance for your reply.