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See the wiki Concept page

Whereas Pub/Sub is many-to-many, one-way transport, Services are one-to-one, two-way transport providing the Request/Reply pattern.

The page you are refering to shows how the actual connection is set up. Basically, this describes that each publisher/subscriber has to register at the master, but the actual topic transmission is happening between the two directly. However, any number of publisher/subscribers to one topic can exist. However, you can only have one service server (i.e. service provider) for one type (name) of a service.