Who should be clicking this checkbox?
Salesforce introduced new API end point for authorizing requests for salesforce vendor and technology partners so if you are and some breaking changes to existing swagger endpoint for consulting partners. A new endpoint to access sandboxes and swagger has been introduced as following.
https://admin.us03.dx.commercecloud.salesforce.com/#/
In order to overcome this breaking change if you are an existing partner and not a salesforce client you need to click this checkbox . on portal when trying to log in
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Also you in your API you configuration you will need to add additional endpoint so as following For example
https://odsdashboard.royalcyber.org/openAuthRedictHandler
https://odsdashboard.royalcyber.org/openAuthRedictHandler/vendor
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Note: Please note you will need to check this checkbox only if you want to access sandboxes for consulting partners, for regular clients do not click this checkbox and this will keep working as is.