Concepts, islands, and archipelagos
The heart of a true semantic web is the concept or a collection of concepts. The heart of the Semantic Web is the resource which is represented by a URI. A resource can be anything. It can be a document on the Web, a reference to a physical real-world object or phenomenon, or even an idea or abstract concept. The Semantic Web itself does not recognize concepts per se, only resources that users associated with concepts in their minds. So, if we want to refer to a concept in the Semantic Web, we need to assign it a resource and URI.
A given application domain, such as astronomy or auto repair or health care, would encompass a collection of concepts. The users of a given domain would need to agree on the terms to be used for the concepts and how they are mapped to resources and URIs. In other words, for a given application domain, the users share knowledge of the resource URI to be used for each concept in that domain.
Different domains may or may not have different users and they may or may not have different concepts, but the users of different domains are free to assign terms and concepts and resources and URIs differently than how they are assigned in other domains. Sometimes concepts in different domains will be distinct and separate and sometimes they will overlap or even be virtually identical.
Different organizations or groups may also have their own distinct concept and resource mappings for a given domain, so that there may be multiple mappings for the same domain concepts to different URIs.
There is no requirement that all concepts in an application domain be present in a given concept and resource mapping. Sometimes only a subset is needed. Sometimes it might be too impractical to represent all concepts.
Granted, there are clearly benefits to agreeing to share concept and resource mappings for each domain, but there are sometimes benefits to having the freedom to exercise full control over the mapping.
Ultimately, each concept and resource mapping for a given domain is essentially an island in this "sea" called the Semantic Web. If coherent and constructed well, call it an island of excellence. Anyone can visit and utilize the resources of a given island, but only to the extent that they agree to accept the concept and resource mappings.
Each island is a land to itself, but sometimes it makes sense for two or more islands to interact and define and make use of shared concepts and resources. These distinct islands may not share and agree on all of their concepts and resource mappings, but enough to make a collaboration of some sort worthwhile. We can think of these collaborating islands as an archipelago.
There may be many archipelagos in the "sea" of the Semantic Web. An unlimited number of them may also choose to share subsets of concept and resource mappings. Sometimes it will make perfect sense to have very large archipelagos, while at other times smaller island groups or even single islands may make perfect sense. Sharing concepts and resource mappings can present many valuable opportunities, but sometimes sharing can be a significant burden or maybe not even be practical at all.
But unless each island is truly "excellent", connecting them together in a network would be futile.
In short, constructing a Semantic Web means carefully mapping concepts to resources, constructing domain islands of excellence, and then interconnecting those domain islands of excellence so that collaboration is enabled and empowered.