How do beta.element.io and develop.element.io differ?

[RokeJulianLockhart]

I noticed in a PR that a user had recorded themselves registering at beta.matrix.org. Attempting to visit it redirected me to beta.element.io, which unlike develop.element.io and app.element.io, references the https://beta.ems.host/ homeserver, instead of https://matrix-client.matrix.org/ for both of the aforementioned alternatives. No develop.matrix.org subdomain exists, as expected.