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

I think, they are in http://opengrok.libreoffice.org/xref/core/vcl/source/control/field.cxx#1114. Internal unit in Writer is “twips”, with 1twips=1/1440 inch. But I’m not sure, therefore I put this as comment, so that the question still shows up as ‘no answer’.

@Regina: Thanks! (Despite the fact that most of the table your link is pointing to, looks contradictory to my poor mind.) Nothing in the ‘char’ column is matching any of my experiences. However, the table seems to back my idea based on unspecific memories that the primary “atom” of ‘Length’ was 0.01 mm in LibO. The ODF specifications (Part 1) “18.3.18 length” only refer to XSL specification (and thus allow for next to everything except the metric point of 1/2660 m).

@Regina: Thanks! (Despite the fact that most of the table your link is pointing to, looks contradictory to my poor mind.) Nothing in the ‘char’ column is matching any of my experiences. However, the table seems to back my idea based on unspecific memories that the primary “atom” of ‘Length’ was 0.01 mm in LibO. The ODF specifications (Part 1) “18.3.18 length” only refer to XSL specification (and thus allow for next to everything except the metric point of 1/2660 m).