Can I use dots for subdomains?

There is no original meaning or exact correct answer to the term subdomain because the usage is blurred by context.

For example, if the nuance of Domain = Desired Units (levels such as example.co.jp, example.com, etc.) is strong, you can also view all subdomains as subdomains.I can do what I can, but I don't think meta.ja is a subdomain of stackoverflow.com.

meta is commonly referred to as a subdomain of ja.stackoverflow.com (meta subdomain of ja.stackoverflow.com), so you may have a strong feeling that meta is the one directly below.

(In the context of host name + domain, meta in meta.ja.stackoverflow.com is the hostname, not the domain name.)

I think it's better to keep in mind the concept that the domain name is hierarchical, separated by meta and ja and arranged from bottom to top by . and not be too particular about the terms themselves.

In the sense of domain hierarchy, ja.stackoverflow.com is a subdomain of stackoverflow.com, not just ja to be a subdomain.Similarly, meta.ja.stackoverflow.com is a subdomain of ja.stackoverflow.com.

[Additional note] When I reviewed the article on the Internet after writing the answer, I found a lot of articles saying ja is a subdomain."Even if the usage is a little different from the original meaning, I think the idea of ""subdomain and subdomain"" will be understood, so I will leave the answer as it is."