It is not one of Wikipedia's policies or guidelines; rather, its purpose is to explain certain aspects of Wikipedia's norms, customs, technicalities, or practices. It may reflect differing levels of consensus and vetting.
Sandboxes, such as a user's sandbox or the more frequently edited Wikipedia:Sandbox, are pages designed for testing and experimenting with wiki syntax and the VisualEditor.
If you have an account, you can create your own sandbox in your userspace. You can find your user sandbox here. If you don't have an account, you can create your sandbox here. (And if it doesn't exist yet, feel free to create it!) For easy access, there is a link labeled "Sandbox" at the top right of every page on either a user icon menu or between the "Talk" and "Preferences" links. You can also put {{My sandbox}} on your userpage.
Decide on a name for the subpage, for example: sandbox2
Enter the following in the Search box, using your actual username: User:Your username/sandbox2
Then click on the reply, "Start the User:Your name/sandbox2 page."
Once the page is created, adding the {{user sandbox}} template onto the subpage designates that page as another sandbox. Unlike your primary sandbox, Wikipedia does not automatically keep a way to link to these, so be sure either to bookmark it or add a link to it on your userpage; you can also use the Special:PrefixIndex special page to display all the subpages of your user page, including sandboxes.
Sandboxes in other languages can be found in a list on the lower left of the English Wikipedia Sandbox. Although using sandboxes in other languages may be useful on certain occasions, it is important to keep in mind that, unless interwiki links are used, each Sandbox can only access the links, images, and templates on the Wikipedia in which it is located.
Bots
Some sandboxes are set up to be automatically cleaned by Cyberbot I, while some automatically have the sandbox header template re-added by Hazard-Bot.