A POS, or "Point of Service" plan combines some of the features offered by HMO and PPO plans.
As with an HMO, members of a POS plan are required to choose a primary care physician (PCP) from the plan's network of providers. Services rendered by your PCP are typically not subject to a deductible. Also, like HMOs, POS plans typically offer coverage for preventive care visits.
Typically, however, you will only receive a higher level of coverage for services rendered or referred by your PCP. Services rendered by a non-network provider may be subject to a deductible and will likely only receive partial coverage. If services are rendered outside of the network, you'll likely have to pay up-front and submit a claim to the insurance company yourself.