What type of network is described as one where costs are covered inside but not outside of its network?

The correct choice is HMO, which stands for Health Maintenance Organization. This type of network operates on a model where members are encouraged to use a designated network of healthcare providers. Costs are generally covered for services rendered within this network, promoting preventative care and primary care management.

When members seek care outside of the HMO's network, they often face significant out-of-pocket expenses, or the services may not be covered at all. This structure emphasizes the importance of using in-network providers to ensure that insurance benefits are maximized and costs are minimized for the patients.

In contrast, other types of networks, such as PPOs (Preferred Provider Organizations) or POS (Point of Service) plans, offer more flexibility for patients to seek care outside their networks, although often with increased costs. Thus, the HMO's strict in-network coverage is what distinguishes it from these other options, making it the correct answer in this context.