How is "character unit" defined?

Question

The OOXML standard (part 1) uses the term "character unit" in various places, for example in the definition of attribute firstLineChars. But it does not define the term "character unit". The file [MS-DOCX] has neither a definition.

CSS has for <length> several font related units, see https://developer.mozilla.org/en-US/docs/Web/CSS/length#relative_length_units_based_on_font

What does Word use, especially when considerate non-Latin scripts?