The grouping of keys in each node is used to minimize the disk access time. The size of the group defines the order of the B-tree. The keys are used for comparison and can be page numbers in a disk storage system. Let dataL be the element in this node, and let dataL.key be its key. Let dataL and dataR be the two elements in this node.
All external nodes are at the same level.
Example in Figure 10.14, page 568 External nodes are introduced only for ease of communication. The book has some great pseudocode and design for this lab.