Furthermore to choosing a top-level domain (TLD) as a main keyword the question at hand is to ponder on the choice of a keyword as subdomain. Of course we all know that Google does and must take into account all information made available when a site is released and crawled by its robots. The question of course is to understand the weight of every single criteria Google uses.
Knowing the content of a page, the URL structure, format, and content, it all needs to be taken into account. Now, having in a domain or subdomain a direct correlation between the keyword and the content of the page is an indication of relevancy therefore somewhat does have importance.
Will this mean it is something to take into consideration, of course, but always keeping in mind that it is a minor criteria in the big scheme of things.
Domain Name System (DNS)
In the DNS tree, the subdomain is just a child domain belonging to a parent domain. Basically, the domain represents a “father node” with ramifications to “node sons” or subdomains belonging to it. Only one point separates the subdomain from the main domain. For example, the two child domains windows.domain.com and doors.domain.com are subdomains of the parent domain domain.com.
Unlike the creation of a domain name, that of a subdomain does not require any particular procedure and is carried out directly through the administration interface of the hosting service of the main domain. Search engines like Google, and Bing treat the subdomain as a separate entity from the main site. Depending on the hosting provider, the number of subdomains can be unlimited.
The main interest in the creation of a sub domain is the optimization of the position given by the search engines. In addition, indexing in directories and search engine is also improved thanks to the use of new keywords directly in the url address. The appearance of the site in the search results will be multiple and more likely, because the subdomains are considered as separate domains from the main domain.