I just want to know if letter and word spacing, in QGIS labeling, are expressed in points, and if the value its absolute or relative to the font.
I have searched a lot, but I can't find any clarification about it.
Both letter and word spacing units correspond to font size units. So, if you set the font size to map units, your letter and word spacing units will be map units.
As you said, that's not very intuitive, but if you have an idea of functions in programming, you can see in line 2559 of the
QgsPalLabeling class, how the
letterspace is applied: the letter spacing number you set is relative to
fontunits. The same happens with wordspace in line 2545.