Apple's going to need some important information from you if you decide to install the iOS 10.3.3 beta on your iPhone, iPad or iPod touch.
First and foremost, you'll need your Apple ID to get the ball rolling. Apple's beta process requires a developer account or an Apple ID so you'll want to track down your info (or the info of the person you're installing for) before you even try to download the proper files.
There's also a good chance the iOS 10.3.3 beta will log you out of some, or all, of your applications and services. Both Apple and third-party.
So, you'll want to make sure you know all of your passwords and logins before you proceed. Otherwise, you might find yourself wasting a ton resetting your various passwords and accounts.