This atlas format is actually the libGDX atlas format, but almost all Spine runtimes that can use atlases use it.
index is used by libGDX for image sequences.
0 means it's the first frame in a sequence.
-1 means it's not part of a sequence.
At least, that's what I understand from it.
Currently, this isn't used by Spine. But it's there because it's part of the libGDX atlas spec.
Spine atlas doc: Atlas export format