KnownSourceFile initializer arguments

What is the significance of the BasicMacroExpansionContext.SourceFileSyntax initializer arguments in:

 let file = BasicMacroExpansionContext
                  .KnownSourceFile(moduleName: "TestModule", fullFilePath: "test.swift")

Are they arbitrary, or is there a meaning to them. For example when I first came across this, I thought I was supposed to give a real module (not sure which), and a full path to the actual test file. Given that the above works, this is obviously not the case. Can anyone enlighten me?
Thanks in advance