For ease of use of directories (reference books), their elements have a hierarchical structure (hierarchy tree). In this way, you can systematize a large amount of information, facilitate the search for objects in the list and speed up work with the system (you can change the properties of grouped elements in one action).

The hierarchy tree consists of main groups, which can contain directory items or other groups of items that will form the second level.

For example, the main groups are Soft Drinks, Cold Appetizers and Hot Appetizers, thus creating the first level. The subgroups Tea and Coffee are on the second level, the Soft Drinks group is their parent.

В одних справочниках количество уровней вложенности иерархической структуры может быть любым, в других оно ограничено. Например, справочник Налоги имеет один уровень вложенности – каждый налог содержит ставки. Справочник Классы ставок не содержит ни одного уровня вложенности. In some directories, the number of nesting levels of the hierarchical structure can be any, while in others it is limited. For example, the Taxes directory has one nesting level – each tax contains rates. The Rate Classes directory does not contain a single nesting level.