7-Mode differentiates from Cluster-Mode. With ONTAP 8, NetApp "merged" the code for what had been two seperate offerings - ONTAP, and the GX cluster solution. Cluster-Mode is basically the latter, and 7-Mode is the former (so called because it basically looks like version 7). The understanding is that eventually 7-Mode will go away, and Cluster-Mode will be the only thing available - but I don't know that they've even come up with a timeline for that.
When you install ONTAP 8, you choose which mode you want to use. That will dictate whether you use 7-Mode or Cluster-Mode docs, software, etc.
You should be able to find lots of stuff about this on NetApp's sites.