Here's how it seems to work. If you have a local repo and want to connect it to github, you would use the "URL" option and put in the github address to set that as the remote repo for your local repo. The first time you do a pull or push on a local branch, it will ask which remote branch you want to use and that becomes the remote location to which you will pushing and pulling changes.
The GitHub.com option only comes into play after you have set the remote repo using the "URL" option. If there are multiple collaborators on a project and someone has created one or more forks of the project, then instead of "No Match" under the GitHub.com, you would see a list of all the forks of the project on which you are a collaborator.
You could then add the forked repos as the 2nd, 3rd, etc. remotes, and any activity on those forks would be displayed to you on the commit graph. This allows people to see what's going on in the forks, and you can drag and drop from the forked parts of the graph to create pull requests to the repo you first set up using "URL".
It's a nice feature, but the remote dialog could be labelled more clearly so that people would know the GitHub.com option was for adding forked repos to the commit graph display.
You must always use the "URL" option to add the 1st remote, and then the GitHub.com option only becomes available after that if there are forks to which you have access.
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