Release candidate is a version that will soon be released as a new stable version. For example, before 1.7.0 is released, there is 1.7.0-RC version, which is a release candidate – it has all new features and is available for testing.
How to switch existing installation to release candidate
Run those commands in webui directory:
git switch release_candidate
git pull
How to switch back to stable version in master branch:
Run those commands in webui directory:
How to get release candidate in a new webui installation
Run those commands (this will create a directory called webuirc – you can use a different name, and you can also rename the directory afterwards):
git clone https://github.com/AUTOMATIC1111/stable-diffusion-webui.git webuirc
cd webuirc
git switch release_candidate