Where are the fake units 'character' (ch) for indentation and 'line' for spacing specified?

In specific the 'character' used as a unit should be judged unusable for decades now. The help doesn' tell anything about the meaning. From experimenting I guess 1 ch = 0.3704 cm, and thus a constant length. Is this correct?
Is 'line' actually used in the sense of "the maximimum height of any part of the line immediately above, including standard descender and ascender height", or something like that? Is it simply "left to implementation"? Is it a constant length again?
Respective question for 'Column Width' and indentations in Calc

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