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Agree with @Rahul Jindal [MVP] Essentially, the Configuration Manager client has logic that looks at several factors, including being able to resolve a management point and the internal domain, to determine whether or not they are on the Internet.
When these factors are not met, the client will evaluate as IsInternet=1 and will switch to Internet mode and attempts to contact the site via the CMG.
When a client comes to onprime, it is likely that the client will meet enough criteria to consider itself IsInternet=0, then it will switch to Internal mode and contact the on-premises SCCM server.
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