The account you use in Microsoft 365 or Office can be a new or existing Microsoft account, or an account assigned to you by your work or school. Many people have both types. Which one you use depends what you're trying to do, but in most cases you need an account to:
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Install and activate Microsoft 365 or Office With many newer versions of Microsoft 365 or Office, your account shows who the Microsoft 365 or Office license belongs to. You use this account to sign in to Office on your devices.
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Manage Microsoft 365 or Office Sign in to microsoft365.com anytime you need to install or reinstall Microsoft 365 or Office, update your security settings, account profile, and if you have a subscription, your payment and billing information.
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Access your email and other cloud services You might also use this account to access your Outlook email, calendar, and contacts, or any files you saved to a Microsoft cloud-service such as OneDrive, Teams, or SharePoint.
Note : Microsoft 365 apps do not support logging in with multiple user accounts from a same Microsoft 365 org or tenant at the same time. You need to log off from one user account to sign in with the other user account.