[HarfBuzz] Beginner question: What are cluster levels?

[Deepak Jois]

In an off-list discussion with Khaled Hosny regarding a specific case
of shaping text with Harfbuzz, he suggested setting cluster levels to
a certain value to improve handling of values returned by Harfbuzz.

I can see that the code has three possible values for cluster levels:

*  HB_BUFFER_CLUSTER_LEVEL_MONOTONE_GRAPHEMES = 0
*  HB_BUFFER_CLUSTER_LEVEL_MONOTONE_CHARACTERS = 1
* HB_BUFFER_CLUSTER_LEVEL_CHARACTERS = 2

I looked at the docs, and there doesn’t seem to be any kind of explanation.

So my question is:

1. What exactly are cluster levels?
2. What do the above values mean?

Thanks
Deepak