Google's multiple sign-in allows users to login to multiple Google accounts at the same time. This means that you can login to one account to access emails while you can see Google calendar configured in your another account, without logging out from the email account. Detailed information on Multiple Sign-in from Google is available here. Note that Multiple sign-in works with only limited number of Google's products, as mentioned in the above link.
When multiple sign-in support is enabled in QuickLogin, the extension will show the multiple-sign-in icon ( ) near the configured accounts, when the extension is accessed from the extension bar. Clicking this icon will attempt the login to the desired account, without logging you out from any of the logged in accounts. This is the same behavior as that of the Google's multiple sign-in functionality.
The advantage that QuickLogin have, compared to the Google's default functionality, is that Google tries to log you in to the same service (Ex: Gmail) while QuickLogin can take you directly to the Product URL OR to the products listing page, as configured in the extension.
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