12. The method of claim 2 , wherein: at least some of the plurality of semantic classifications indicate a classification of a role of content relative to a content unit from which the content was determined; storing the plurality of annotations comprises storing a third annotation relating to the first content unit, the third annotation comprising a third semantic label and third content, the third semantic label indicating a role of the third content relative to the first content unit; and wherein storing each of the plurality of semantic labels in the first table comprises storing the third semantic label in the first table.
12. The method of claim 2 , wherein: at least some of the plurality of semantic classifications indicate a classification of a role of content relative to a content unit from which the content was determined; storing the plurality of annotations comprises storing a third annotation relating to the first content unit, the third annotation comprising a third semantic label and third content, the third semantic label indicating a role of the third content relative to the first content unit; and wherein storing each of the plurality of semantic labels in the first table comprises storing the third semantic label in the first table. 13. The method of claim 12 , wherein: the third annotation is a structural annotation identifying a structural element appearing in text of the first content unit; the third content of the third annotation identifies the structural element; and the third semantic label identifies a structure type of the structural element.