The purpose of this document is to assist you in understanding when and why you should consider a subdomain.
HubSpot: Subdirectories represent levels of pages within each subdomain of your website. For example, if you have a blog which is located on your www. domain at www.yoursite.com/blog, www. is the subdomain and /blog is the subdirectory. A blog post example would look like this: www.yoursite.com/blog/blog-post-article.
Alternatively, you can set your blog as its own subdomain. Instead of www. you could set your subdomain as blog. or news., for example. A blog post URL would look like blog.yoursite.com/blog-post-article. You'll notice that there is no /blog subdirectory. In this case, you don't need that extra level of hierarchy since you've set blog. as the subdomain.
Our Recommendation: If you've already set up your website with subdirectories, stick to that format. For SEO reasons, the least amount of change to URL's and sitemap structure the better stability you'll have with your ongoing SEO results, ranking etc.
If you're considering creating a new subsection of your business then a subdomain is the way to go. Remember Subdomains need to be tracked separately in Search Console, they are treated as a unique site in itself.