Hi, welcome.
1) studios they can request from is different (studios may change by time) *.
2) Premium requests will be fulfilled for sure (only C4S studios has a chance to get ban)
* For free users:
Allowed stores: Clips4Sale, Modelhub, APClips.com, MFC Share
Forbidden stores: IWantClips, ManyVids
* For premium users:
Allowed stores: Clips4Sale, IWantClips, ManyVids, Modelhub, APClips.com, MFC Share
Forbidden stores: -