YouTube For Artists is YouTube's own analytics site which tracks song plays for an artist across all uploads on YouTube (instead of just the Artists YouTube channel). One example could be if a popular YouTube channel uploads your song at it receives 700,000 views, this is counted even though the video is not hosted on the Artists own YouTube channel.
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Currently, YouTube doesn't have an application process for access and they add artists when they choose too. If YouTube offers a specific sign up form or process in future we will share that link.