Does anyone know when to you use '+' or '.' when accessing enum values (or other class members perhaps)?

Example:

To get the documents and settings folder from powershell, you do something like:

[System.Environment]::GetFolderPath([System.Environment+SpecialFolder]::MyDocuments)

SpecialFolder is a .NET enumerator, but it can't be accessed by [System.Environment.SpecialFolder]::MyDocuments syntax...

On other hand - a member of the SearchOption (System.IO namespace) enumerator is accessed by:

[System.IO.SearchOption]::AllDirectories

Does anyone know what is the difference?