What is the importSources parameter in RazorProjectEngine.Process?

[Merlin04]

I'm using RazorProjectEngine like this:

RazorProjectEngine engine = GetRazorProjectEngine("insert namespace here");
string fileName = Path.GetRandomFileName();
RazorSourceDocument document = RazorSourceDocument.Create("Insert template source here", fileName);

RazorCodeDocument codeDocument = engine.Process(
    document,
    null,
    new List<RazorSourceDocument>(),
    new List<TagHelperDescriptor>());

RazorCSharpDocument razorCSharpDocument = codeDocument.GetCSharpDocument();

SyntaxTree syntaxTree = CSharpSyntaxTree.ParseText(razorCSharpDocument.GeneratedCode);

When I call engine.Process there are four arguments - source, fileKind, importSources, and tagHelpers. I know what source and tagHelpers are, but I don't know what fileKind or importSources are. I tried to find documentation on them but was not successful. What do these parameters do? I'm more interested in importSources as it sounds more useful.