What's the meaning of hash sign (#) in SPARQL? -

in sparql, see usage of # @ end of prefix definitions, this:

@prefix dt:   <http://example.org/datatype#> 

what's purpose? couldn't find in sparql documentation.

your example seems in turtle, in sparql syntax be:

prefix dt: <http://example.org/datatype#> 

but it’s same idea: instead of having use full iris in query, can use prefixed names:

1. in example, prefix label dt. it’s mapped iri http://example.org/datatype#.
2. in query, might used dt:foobar, foobar called local part.

3. the mapped iri prefix label , local part concatenated form "actual" iri:

   http://example.org/datatype# + foobar =
   http://example.org/datatype#foobar

4. (instead of using dt:foobar, use <http://example.org/datatype#foobar>.)

so # happens part of iri design. it’s popular way structure vocabulary iris in semantic web. other popular way using /. see hashvsslash.