What does L4_Sigma0_GetPage_RcvWindow?

I haven't found any documentation about the sigma0 functions, neither in
the reference manual, nor in the header file sigma0.h itself. Maybe you
could add that in the next revision of the reference manual? But for now
I would be quite happy, if someone could explain what

L4_Fpage_t L4_Sigma0_GetPage_RcvWindow(
  L4_ThreadId_t s0,
  L4_Fpage_t f,
  L4_Fpage_t RcvWindow
)

does or point me to a document that explains the sigma0 helper
functions? My understanding is: Tell s0 to map "f" in his address space
to RcvWindow in my address space. The return value should then be within
RcvWindow. If s0==NilThread ask sigma0 for the page.

Thanks,

Martin.