Socialization can be lumped into 2 categories: active and passive.
Active socialization includes that in which the dogs actually meet and play together, like social hour or play dates. Passive socialization is where dogs are around each other, but do not actually interact.
Both types are a great option (and are great when used in conjunction!), although they serve two entirely different purposes. Active socialization teaches dogs how to interact and identify social cues, while passive socialization allows dogs to learn to be comfortable around each other while focusing on their handlers.