explain local version: How to understand section "Estimated resources for plan:"

I have the following info in "Estimated resources for plan:".
Scratch Memory MB: 897
File Handles: 1005
Worker Threads: 24
Blocking Threads: 75
Externalizing Ops: 0
Unbounded Mem Ops: 0
Max Threads: 16

what's the meaning of Unbounded Mem Ops and Externalizing Ops? And there are 3 kinds of threads, what's the relationship among them? Worker Threads,Blocking Threads and Max Threads