RAIN? RAIN is a reference to an old Centera term, it has no relation to how Isilon protects data.
It stood for:
So you would say that data on Centera was protected with RAID (within each node) + RAIN (vertically across nodes), with either CPM or CPP protection (Mirroring or Parity).
All of that said, data on Isilon is protected with Erasure coding. To learn more about erasure coding, look up Reed-Solomon and/or Erasure Coding. You might want to take an Isilon basic admin class to learn more.