Feel free to tell me this isn't the proper place for my question but given that networking and DNS are hand in hand I thought it might be reasonable to ask here.

In working with several OSINT sources for domain processing it seems like the way domains and subdomains are processed essentially equates subdomains with FQDNs.

For example, several APIs (and even ChatGPT) classify the following:

access.api.bbc.com

account-api.api.bbc.com

account-api.int.api.bbc.com

account-api.stage.api.bbc.com

account-api.test.api.bbc.com

account-cdn.test.api.bbc.com

with subdomains as either:
all subdomains as api.bbc.com 

or as subdomains of access.api, account-api.api, account-api.int.api, etc. 

instead of classifying as:
api.bbc.com
int.api.bbc.com
stage.api.bbc.com
test.api.bbc.com

Has this become common practice? Is there a definitive way to determine subdomains? I seem to recall that "older" dns server software wouldn't allow this but it could be that my memory is faulty.

Thanks!

Cheers,
Harry