Test Server
From Legacy Roblox Wiki
Roblox has a very fast update schedule. Every 2 weeks we make a new major release, and in order to make sure the program works well we test things with the users.
Users that know what they're doing are welcome to join us on the test server at gametest and sitetest. Sometimes when we want to test a new feature, we'll put it out onto the Test site. It's simple to get started, just like on the regular Roblox site you run the Install process, just on the test site instead! Once you get the Test version of Roblox installed you can start helping us out testing new features.
When you want to go back to normal Roblox, just go play a game on the main site. That will put your version back to the current version.
