The notion of a graph type is related to Resolvers; see also this thread in the mailing list archive.

Graph types:

      This is the default triple store graph. Specifying this type is equivalent to omitting the type parameter.
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      A view graph that represents a combined set of graphs. See the Views section for more information.
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      A full-text string index graph based on Lucene. All statements placed into a full-text graph must have a literal-valued object. Queries against a full-text graph result in a match if the object in the query constraint is similar to any stored statements, rather than needing to be identical. See the Full-Text Graphs section for more information.
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      A datatyping graph that represents the property of orderedness that literals like dates and numbers possess. Datatyping graphs are used to constrain results to be above, below, or between specified dates or numbers. See the Datatyping Graphs section for more information.
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      A nodetyping graph that represents the type the RDF nodes have in a graph. Nodetyping graphs are used to constrain results to be literals or URI references. See the Nodetyping Graphss section for more information.
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      A filesystem graph that can be queried to return information about files and directories on filesystems. See the Filesystem Graph section for more information.
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      A graph containing virtual statement about the characters prefixing string literals, or URI references.