In 18C it should be possible to upload RDF files in Turtle (TTL) format - but what is the purpose of the PREFIX line below (from the Manual) as the Turtle file itself contains one or many PREFIX definitions?
That's a good question. This is most likely a copy-paste error in the user guide. PREFIXES in the SPARQL update statement are not used when parsing the turtle file. The PREFIX definitions in the turtle file itself will be used instead.