I understand that:

• If you enable the 'Manage multiple companies' option from Settings --> Configuration --> General Settings you can manage multiple companies from a single odoo database.
• Then for every user (in Settings --> Users) you can choose the company that this user works for (through the "Company" field). Here you cannot choose a company which is not listed in "Allowed Companies" for that user. (As explained here )

What I don't understand is this:
I did some tests and I noticed that a user can view the Customers, Projects etc of a Company X if he works for that Company X or if his company is 'Parent Company' of the Company X. Changing the "Allowed Companies" field didn't have any other effect.

Is there any other usage of the option "Allowed Companies"?