"Significant characters" in Path.GetFullPath

[`Path.GetFullPath`](https://learn.microsoft.com/en-us/dotnet/api/system.io.path.getfullpath?view=net-8.0) is documented in part as follows:

> If a path contains no significant characters, it is invalid unless it contains one or more "." characters followed by any number of spaces; then it will be parsed as either "." or "..".

As far as I can see, the term "significant character" is not defined anywhere, either in the docs or apparently on any of the Internet (from thirty seconds of Googling); I believe one must read [the code](https://github.com/microsoft/referencesource/blob/51cf7850defa8a17d815b4700b67116e3fa283c2/mscorlib/system/io/path.cs) before finding any more references to this term.