Represented as an ordinal number followed by either a single. 0 or a set of ordinal numbers separated by dots, the node data type corresponds to a position in a tree structure. 节点数据类型用一个序数加一个.0或一组以点号隔开的序数来表示,它对应于一个树结构中的一个位置。
Default values must be of ordinal, pointer or small set type 默认的值必须为序数、指针或小集类型