Short answer: A POS plan is a hybrid: in network it works like an HMO (with a PCP and referrals), but it also covers out-of-network care like a PPO, at higher cost.
A Point-of-Service (POS) plan blends HMO and PPO features. In network you choose a primary care physician and use referrals to get the lowest cost; out of network you still have benefits, but at higher deductibles and coinsurance. POS plans appeal to members who want managed, low-cost in-network care most of the time but value the option to go out of network occasionally.