Undertow threads vs Xnio worker threads

Question

I am trying to understand the difference between

Undertow.builder().setIoThreads(0);
Undertow.builder().setWorkerThreads(0);
from this package:
io.undertow.Undertow

and

Xnio.getInstance().createWorkerBuilder().build().getOption(Options.WORKER_IO_THREADS)
Xnio.getInstance().createWorkerBuilder().build().getOption(Options.WORKER_TASK_CORE_THREADS)
Xnio.getInstance().createWorkerBuilder().build().getOption(Options.WORKER_TASK_MAX_THREADS)
from these packages:
org.xnio.Options;
org.xnio.Xnio;

so the undertow builder has IO threads but so does the worker, and the undertow builder has worker threads but how does that correlate to the worker thread options?

additionally both of these can be set in spring boot via application yml so they must be different right?

server:
  undertow:
    threads:
      io: 0
      worker: 0
    options:
      socket:
        worker-io-threds: 0
        worker-task-core-threads: 0
        worker-task-max-threads: 0