DMARC's policy specified at your main domain (organisational domain) takes care of and applies to subdomains by default as well. Therefore, If you have added your main domain to OnDMARC but send emails from subdomains of that domain as well then the reports will show results that cover you main domain and subdomains.. 

If you would like to see separate reports for your subdomains you will have to add them to OnDMARC. Whenever you send emails with the From address being @sub.domain.com then you will see separate reports for that subdomain.

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