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why I am able to create roll up summary on account to Opportunity.
The relation between Account and Opportunity is "Lookup", but still you are able to create roll up summary field on Account. Why?
This is one of those functionality which was delivered some time back and is out of the box between the Standard Objects like Account-Opportunity, Account-Case, Contact-Task.
Please have a look at the below this idea link:
https://success.salesforce.com/ideaView?id=08730000000BraS
It was delivered almost 10 years back and is something unusual. Keeping the business requirement in mind it was delivered.
Let me know if it clears your doubt or you need some more explanation.
Thanks & Regards,
Jainam Contractor
All Answers
Do you have Advanced Multi-Currency enabled on your SFDC.Org? I'm pretty sure that AMC$ kills all Currency Roll-Up Summary fields.
Thanks,
Rakshana
This is one of those functionality which was delivered some time back and is out of the box between the Standard Objects like Account-Opportunity, Account-Case, Contact-Task.
Please have a look at the below this idea link:
https://success.salesforce.com/ideaView?id=08730000000BraS
It was delivered almost 10 years back and is something unusual. Keeping the business requirement in mind it was delivered.
Let me know if it clears your doubt or you need some more explanation.
Thanks & Regards,
Jainam Contractor