What does "upstream" and "downstream" mean?

I saw these terms are being used in the Rancher docs. However, it’s not clear to me what do they mean. I’d appreciate if someone could explain this. Thanks.

Hi, upstream cluster means the cluster on top which your Rancher is provisioned. And downstream clusters are the clusters you have provisioned or imported on your Rancher Manager.

If you navigate to the cluster management page, you will find a “Local” cluster. This is your Upstream cluster. Any other cluster you provision using Rancher or import existing cluster on Rancher would be a Downstream cluster.