The decision between a subdomain and a subdirectory concerns where a blog, knowledge base, language version or separate section of the website will live. Search engines can handle both solutions today, but in practice a subdirectory more often delivers a better result, because the content contributes to the authority of the main domain and does not need to build its own. A subdomain is worthwhile when it involves a project that is separate both in content and technically, when it runs on a different platform, or when security separation requires it. The decision is hard to change once made, because moving means migration with redirects and a temporary dip. For language versions, it also matters whether the company targets individual countries with separate domains. If a company is undecided, a subdirectory tends to be the safer choice, because it does not require building authority from scratch and it is managed together with the main website.
See also: Domain Strategy for Markets, Canonical domain version, Hreflang.
