Why do some styles measure font sizes as %, others as points? What determines which?
Could you edit your question to give examples among built-in styles?
When you edit a style, either a paragraph or character style, on the Organizer tab you’ll find the field Inherited from: that set a style from which the current style picks some values: A percentage value for font size means that the font size on your style will be a percentage of the size set on the “parent” style.
For example, you can set a style (let’s say, Heading 1) to use the 150% of the font size used by another style (for example, Heading). If the parent style is set at 10 pts, then the real font size of your style will be 15 pts. If later you change the parent style to be 12 pts your style will then use 18 pts without extra effort.
So, the font size is a percentage only when the style is inherited. I thought I checked that, but maybe I did not.
Thank you for this information. It is just what I need.
@RGB-es: I made a test and saw it works. Since I had trouble in the past with it and avoided the feature, do you know in which version it was fixed?
That feature would be very very useful in character styles if the %-size was applied to the current paragraph style instead of the regular inherited character style. Do you think it could be considered a valuable enhancement though it conflicts with standard inheritance rules?
the font size is a percentage only when the style is inherited.
It is by default a percentage, but you can edit the child style and use points (or other units for some other elements of a style) instead of a percentage. However, in such a case when you change the parent style, the hard-coded values won’t change. And vice versa, you can always change a hard-coded value in a child style to a percentage to make it change in accordance with the parent style.
@ajlittoz I do not remember problems with this option (with others, maybe, but not this one 
). About the enhancement request: YES! I think that an option for a character style to be a percentage of the background paragraph style font size would be quite interesting!
All the explanations for this feature cannot make up for its uselessness. Nobody thinks or writes this way. All we writers want to see is the font size, inherited or not. The percentages are maddeningly useless, time-wasting and frustrating.
LO tries to cover everybody’s needs. You are free to use any unit which makes senses to you. This is why inches and centimeters are offered, points and others. In my case, when I deal with headings I want relative sizes to remains consistent when I change the base size without the need to track related styles. This is where percentage is interesting. If you have no use for it, don’t use it.
PS: this is a Q&A site, not an opinion forum.