Setting up companies hierarchy is a way to configure the issue security levels in accordance with company-contact relations.
Set the parent-child relations between the entries in the Companies directory using the Parent Company and Child Companies attributes.
That will allow you to:
create the child companies right from a company
associate a company with the parent one
There is not possible for now to browse the companies tree, but everyone with access to the Companies directory will be able to browse the parent and child companies of the current one.