Interlinear Annotation
Three formatting characters provide support for interlinear annotation (U+FFF9, U+FFFA, U+FFFB). This may be used for providing notes that would typically be displayed between the lines of other text. Unicode considers such annotation to be rich text and recommends using other protocols for such annotation. The W3C Ruby markup recommendation is an example of an alternate protocol supporting more advanced interlinear annotation.
Read more about this topic: Unicode Control Characters