2.13. Graphical display of language units
Following are excerpts from the Unofficial DVD specifications Guide 2.2 ebook.

...This document uses the convention and graphical symbol to display a language unit as shown in the following illustration, representing its data structure as defined by the DVD specifications. From a hierarchy perspective, a language unit is a level higher than the PGCs. Language units are present in certain domains (domains are explained in the next section of this document), and cannot be present where a title exists. The following diagram shows the relationship between language units and PGCs. More details on language units are explained in the Unofficial DVD specifications Guide 2.2 ...