document doesn't define "modifier key unicode graphemes"

In chapter 14. Element Interaction, section 14.3 Element Send Keys:

cluster is a modifier key

The document refers to ui-events without any other explanation, but ui-events does not define unicode extended grapheme clusters for modifier keys, it defines strings for modifier keys. I have looked around in the text, and I can find some unicode codepoints associated with modifier keys later, but it is not clear whether these are the unicode grapheme clusters in question, and it is not clear which of these unicode characters are the modifier keys the text is talking about (the tables that assign unicode characters to modifier keys also contain a lot of keys that aren't modifier keys).

Start with chapter 14, section 14.3 Element Send Keys, especially the statement that a cluster is a modifier key, then compare its references to UI Events. Clarify which Unicode extended grapheme clusters represent modifier keys and how the later modifier-key tables apply; the section should define or unambiguously reference these terms when done.