At UT, when you are prompted to log into some application or system your login account may take several different forms, but username all will generally be some variation of your EID or official email address, and the password will be the same as your EID password. Very few systems use something completely different from this, and those should explicitly inform you when that is the case.

UT EID - All UT Faculty, Staff and Students have a UT EID (University of Texas Electronic Identifier). This is used for logging onto most restricted web pages and online applications and to the university Wi-Fi network and VPN. You can manage your EID at https://idmanager.its.utexas.edu/eid_self_help. There you can reset your EID password, change some personal information, and upgrade your EID security assurance among other things.
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Austin Account - Your Austin Active Directory Account is used for logging onto computers and accessing certain Windows resources such as Windows Server file shares like Austin Disk. Your Austin AD user name and password are the same as your EID and password, except that the term "AUSTIN\" frequently needs to be entered as part of your login name (please expand the image below to see a screenshot of this). If you use a Macintosh or Linux Computer you might not need to use an Austin AD Account to log onto it, but you may be prompted to use use your AustinAD account when you connect to certain file shares or other network resources.
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Office 365 Logon - Your UTexas Office365 Logon is actually tied to your Austin Active Directory Account, but it uses your email address as the login name instead of your Austin AD user name. This logon is use for all Office 365 services including Outlook email, Teams, One Drive, Share Point, and others. The preferred way to log onto the UTexas UTexas Office365 service is to use https://office365.austin.utexas.edu/. This will let you log on using either your EID or IID (your [EID]@eid.utexas.edu). If you bypass the UTexas login portal and go directly to Microsoft's own Office 365 Site, then you will need to specify your UT email address for it to understand you are logging as part of UT. You office 365 login uses the same password as your EID and Austin AD account.

logon from office365.utexas.edu

logon directly to office365.com

McCombs Tech Wiki - Accounts

Accounts For Everyone

Additional Accounts For Faculty and Staff