My understanding of the terms lookup mechanism is that you can provide a term and then as a result of that surface the documents that match a particular term name. So for example if you had an User and a Transaction, you could lookup the user's name and surface his transactions.
However, if I wanted to surface all user's names across all transactions, I'm assuming that wouldn't be possible with this mechanism. i.e. a table that has both the user name and its transactions.
From reading the docs, I'm assuming that's where nesting/child-parent relationships comes in.