How Text Layout Manager generates Layout Fragment from Text Elements?

During the presentation (19:43) it says the NSTextLayoutFragment 'Layout information for one or more elements'. But from the documentation it seems that you can only generate a layout fragment from one NSTextElement. Besides there is only one property that associates a text element. What does it mean by 'parent element' anyway?

How Text Layout Manager generates Layout Fragment from Text Elements?
 
 
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