Answer Description

This type of network setup is called an Extranet. An extranet is a private network that allows access to certain authorized parties. Most commonly this would be to share systems like a file server between two companies that have created a long-term partnership.

Wikipedia

An extranet is a controlled private network that allows access to partners, vendors and suppliers or an authorized set of customers – normally to a subset of the information accessible from an organization's intranet. An extranet is similar to a DMZ in that it provides access to needed services for authorized parties, without granting access to an organization's entire network. Historically, the term was occasionally also used in the sense of two organizations sharing their internal networks over a virtual private network (VPN).