@graph usage questions

Trying to understand the spec portion about graph.  I see this example:
{
  "@context": ...,
  "@graph":
  [
    {
      "@id": "http://manu.sporny.org/i/public",
      "@type": "foaf:Person",
      "name": "Manu Sporny",
      "knows": "http://greggkellogg.net/foaf#me"
    },
    {
      "@id": "http://greggkellogg.net/foaf#me",
      "@type": "foaf:Person",
      "name": "Gregg Kellogg",
      "knows": "http://manu.sporny.org/i/public"
    }
  ]
}
"The @graph keyword collects such nodes in an array<http://json-ld.org/spec/latest/json-ld/#dfn-array> and allows the use of a shared context."

Does this mean:

1.      Graph content is always an array of nodes?

2.      Or graph content can have just one node (a root node) and its subnodes?

3.      What are other good examples of graph usage?

Thanks,
Radu

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