The meaning of VIRTUAL-RUNTIME_foo confused me a lot at first. I suggest adding the following note to the end of the PROVIDES glossary entry to explain what it means:

  Note {

  There is a corresponding mechanism for virtual runtime dependencies (packages),
  but it does not depend on any special functionality beyond ordinary variable
  assignments. For example, VIRTUAL-RUNTIME_dev_manager refers to the package of
  the component that manages the /dev directory.
  
  Setting the "preferred provider" for runtime dependencies is as simple as e.g.
  the following assignment in a configuration file:
  
    VIRTUAL-RUNTIME_dev_manager = "udev"

  }

Cheers,
Ulf