DMARC record inheritance

Q: How do subdomains benefit from DMARC specified at the top of a domain?

A DMARC record on the top of the domain will be inherited by any subdomain which does not have a DMARC record of its own.
e.g. a DMARC record on dmarcian.com will affect all subdomains of dmarcian.com, UNLESS a subdomain has published its own differing record

Use of an 'sp' tag in the top domain will cause the policy specified in the 'sp' tag to be inherited instead of the default 'p' value. Reporting behaviour remains the same.