[boost] [Filesystem] Native Pathname Format

I can not determine from the Filesystem documentation, nor the reference documentation which comes with it, what is the acceptable syntax for the native pathname format. It appears one must start the native pathname format with //: in order for it to be recognized, but I can not tell from what appears to me to be very confusing documentation whether this is so for the Boost Fileystem implementation. This would mean, let's say under Windows, that "c:\afile" would not be recognized but that "//:c:\afile" would be recognized, as far as I can make out from the documentation. It sure is confusing IMO. Can anybody clarify how this works for Filesystem and/or where there is a clear explanation of how it works in the documentation ?