TargetDescription string

In "Writing an LLVM Compiler Backend", there's some discussion of the
TargetDescription string, but it doesn't explain the examples I look
at. For instance, in the description of the PowerPC, I see


What's "preferred alignment" versus "ABI alignment"?

What are the 3 figures following the "i"? (the documentation suggests
there will only be 2).

I see two entries for "f". Do I read these as info about "f64" and "f128"?

What is "n32:64" about?