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OAuth means the users will have a username and password specific to the ArcGIS Organization. They will be prompted to enter those credentials when they try to sign into their org.
(Note: If a client says they use SAMLonly SAML is used, that is a type of OAuth).
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IWA means “integrated windows authentication”. In this situation, a user’s the credentials are tied in with the account they use used to sign into their a computer. While they there may have to click be a “sign in” button, they you will never be prompted to enter a username or password.
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To determine if a client has an ArcGIS Organization that uses OAuth or IWA, have them open up an incognito window in their a browser, then navigate to their the ArcGIS Organization in question. If they you are prompted to enter a username and password when they sign in, they use it uses OAuth. If they aren’tit doesn’t, it’s IWA.