What does δ < 1.25, δ < 1.25², δ 1.25³ (delta 1.25) stand for?

I read a paper where δ < 1.25, δ < 1.25², δ 1.25³ are given.

Since I do not really know for what they stand for …

My question would be:

  1. For what exactly do the values stand for?
  2. Why do we want to get/compair those values?
  3. How to get those values?

Thanks a lot for your help!

I believe this metric is used in depth estimation use cases comparing the amount of output pixels with a small, medium, and large difference from their ground truth value.
However, I don’t know how these values were defined.

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I additionally found this one:

But I still can’t figure it out, what it means.
Any idea?