Occupancy of a B+ Tree
B+ trees, like all B-tree variants, are typically page-structured. A page size is chosen (which is a multiple of the disk page size and the CPU's page size). Each node or leaf is a page in size. Organising the tree this way eliminates external fragmentation.

We pay for it with internal fragmentation.

Each node and leaf can have a variable amount of data stored inside it. So for if a node is not full, some of that page is unused. When we say that a B+ tree has 67% occupancy, this means that 1/3 of the allocated space is unused memory at the end of pages.