What does XM Ready or Sirius Ready actually mean?

[Turkish]

I see these car stereo headunits with XM Ready or Sirius Ready labels on them and I was wondering what that actually means? Does it mean I can hook up the headunit and I'll have XM or Sirius or do I still need the XM or Sirius units to listen to it (aside from the monthly fees of course)?

 

[Anubis]

it means the headunit can change the source to the XM imputs, so you can connect teh sat receiver to it