If you want to automatically resolve all subdomains of a domain, you can use a DNS wildcard (“*”) record.
However, if you have already created explicit subdomains under a domain, the wildcard no longer applies on that level. An existing subdomain entry overrides the higher-level wildcard DNS record.
In this case, a separate wildcard for the specific subdomain is required to ensure that its sub-subdomains are resolved correctly.
Example
You have created the following DNS records:
* .yourdomain.com
blog.yourdomain.com
You now want to use admin.blog.yourdomainname.com and notice that the wildcard *.yourdomainname.com does not apply, but only works for direct subdomains that are not explicitly defined.
This is because all non-explicitly defined subdomains below yourdomainname.com are resolved via the wildcard, including additional levels.
However, sub-subdomains below blog.yourdomainname.com are not resolved via wildcard, since the subdomain blog already exists as its own DNS entry and does not have its own wildcard record.
If you require wildcard resolution for the zone *.blog.yourdomainname.com, it must be created separately.
Note on DNS standards (RFCs)
This behavior is defined in RFC 1034, Sections 4.3.2 and 4.3.3, and further clarified in RFC 4592, Section 3.3.1.