A subdomain of the domain example.com would be any domain that is prefixed with "example." For instance, if you have "blog.example.com" or "shop.example.com," these are both subdomains of "example.com."
The formatting appears to have a mistake in your question. Generally, the underscore character "_" is not used in domain names; the correct format should use a dot "." between the main domain and the top-level domain (TLD). Therefore, the correct form should be "example.com" instead of "example._com."
If you're looking for specific examples of what could be considered a subdomain of "example.com," you might have:
mail.example.com
store.example.com
test.example.com
Any domain that follows the same format, prefixed by a name and separated by a dot, would be a subdomain of example.com
.