This example demonstrates a nested data join where each parent table header element binds to a sub-array of data, creating child elements with different values. Using D3 v6’s `selectAll().data()` pattern, the code first joins `topData` to header cells, then re-joins each header’s `div.th-inner` with entries from `newData`. Child `div.th-but` elements are appended with checkboxes and spans, each bound to individual sub-data items, illustrating how to manage varying child data per parent.