Which branch does username.github.io point to for documentation?

Hello all
I am using mkdocs to build static files. I wish to use mkdocs gh-deploy -b option to build static files in a separate branch.
Since, I have setup a repo named .github.io according to GitHub pages guide, now I am wondering at which branch should I store my static files? Either master or gh-pages.
In other words, which branch does <username.github.io> point to for hosting static content?

You can refer to the help documentation for the full details, but user and organization pages must publish from the master branch.