"A real-time symbolic processing system on a single processor should have the following four features"
The language they're developing in/for is Lisp, which is used (a lot) for symbolic processing. That's the "symbolic processing" part. "real-time" is the constraint they're trying to achieve and relates to system performance. "single processor" is just another constraint. If you permit multiple processors then some of the problems making Lisp real-time go away or are mitigated as you can do things like shove the GC bit into a second processor and execute it in parallel. Being constrained to a single processor means you can't do this, when the GC is going it's using the full CPU and taking time away from other computations (symbolic processing, in their case).
The language they're developing in/for is Lisp, which is used (a lot) for symbolic processing. That's the "symbolic processing" part. "real-time" is the constraint they're trying to achieve and relates to system performance. "single processor" is just another constraint. If you permit multiple processors then some of the problems making Lisp real-time go away or are mitigated as you can do things like shove the GC bit into a second processor and execute it in parallel. Being constrained to a single processor means you can't do this, when the GC is going it's using the full CPU and taking time away from other computations (symbolic processing, in their case).