#using strAdds the namespace in str into Rowan’s list of used namespaces, which it checks through when attempting to resolve a .Net type reference. A namespace is simply a string which is added to the front of the type name, and is a convenience. For example:
#using "System.Windows.Forms"
$MessageBox:Show("Hello, world" "R" $MessageBoxButtons:OK $MessageBoxIcon:Information)
OK
The System namespace is automatically used.