Hello @Venkata satyanarayana Machari veera (CIS) - Thank you for reaching out to us on Microsoft Q& platform. Happy to help!
I see that you are using azure billing API to fetch data (Storage and Virtual Machine) to understand the data/Storage being consumed and the number of hours Virtual Machine (Compute) resources were running. In general, we recommend that you use the Cost Details API instead of the Usage Details API: https://learn.microsoft.com/en-us/rest/api/cost-management/generate-cost-details-report
Regarding the Quantity column, what you described in your post is correct. You can find some more details here: https://learn.microsoft.com/en-us/azure/cost-management-billing/automate/understand-usage-details-fields
Hope this helps. Plese reply if you Have more questions!
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