What is "hierarchical-data"

A hierarchical database is a type of system for managing a database that performs the connection of records in a structure where the data is organized as branches in a tree, for this, this system makes connections so that each type of record has only one owner.

The hierarchical database is based on a Entity Model and Relationships; where each record is a group (collection) of attributes (fields), and each of which contains only one information; whereas, a link is the association (link) between two records.

An example that can be taken is the case of a commercial database, where a product is owned by a single customer