What is NCCL Window

Accelerated Computing GPU-Accelerated Libraries
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I was reading the NCCL doc User Buffer Registration — NCCL 2.27.5 documentation

NCCL supports window registration, which allows users to register local buffers into NCCL window and enables extreme low latency and high bandwith communication in NCCL.

But what is exactly NCCL window? I didn’t find any explanation of NCCL window in the doc.