With the release of Windows 10, it enabled users to log in using their Microsoft accounts. This gives a has a privacy issue where, in the lock screen of the computer which is accessible by anyone can view the email address that is used to log in. This could lead to unsolicited SPAM emails and other unwanted risks.
By default there is a setting in the sign-in options to turn on or off to show the account details (eg: email address) on the login/sign-in page. By default this option is switched off and the account details are not displayed.
If this option is turned on, your account details will be displayed on the sign in page to the computer when it starts.