It is not for all runs inside paragraph, but paragraph mark.
Find explanation from spec (2.3.1.29 ):
The paragraph glypg's formatting is stored in the rPr element under the paragraph properties(pPr), since there is no run saved for the paragraph mark itself. The paragraph glypg is used to represent the physical location of the paragraph mark for this paragraph.
Besides, OpenXmlClassesExplorer is a good tool for looking up schema element/SDK class definition in a easier way compared to searching in spec.
Hope this helps.