The "Inherit From Primary Admin" user option gives users automatic implicit access to any companies that have the user's primary company set as the administrative parent.

For example, given the following:

Name	Type	Admin Parent	Reference ID
Big Corp Personalisation	Admin	N/a	10000
Big Corp Retailer A	Retailer	10000	10001
Big Corp Retailer B	Retailer	10000	10002

A user with "Big Corp Personalisation" set as their primary company along with the "Inherit From Primary Admin" will automatically have access to "Big Corp Retailer A" and "Big Corp Retailer B".