Is it better to use a subdomain or a directory for a new content?
This question is very common during the launch of a blog. Indeed, it is possible to base your work on these 2 models:
- example.com (subdomain)
- example.com/blog/ (directory)
This choice in not insignificant when it comes to your natural referencing.
As for a directory, a link pointing from your blog will be considered as an internal link. In the case of an installation on a different subdomain, then it will be a backlink.
However, we can legitimately think that this backlink will be less weighted as a "traditional" backlink". It is therefore pretty easy for a search engine to infer than links coming from a subdomain are very simple to get. They actually are not a recommendation as strong as a backlink sent out by a third-party website.
The popularity of your blog will be impacted by this choice, too. If it has been installed on a directory, then your blog will be able to totally benefit from the authority and the popularity of your domain, which is probably less true for a subdomain.
Please note that you will have to both popularize (in terms of links) your subdomain blog.example.com and www.example.com.
This duty can be an opportunity as well as a burden. This will depend on your available resources, thus there is no systematic answer to that question.
Finally, the handling of the content and links of a subdomain in relation to the main domain may change depending on time or updated from Google.