When does using a subdomain make sense for SEO?

Short answer: A subdomain makes sense for parts of a site that genuinely separate from the main property, technically or functionally: an application on different infrastructure, a product line with its own positioning, or large page clusters that should not burden the main site. Because authority signals largely accumulate separately on a subdomain, the subfolder remains the default choice.

Cases where a subdomain is justified

• Technical necessity: an app, documentation, or community forum running independently of the main CMS.
• Brand separation: a product aimed at a different audience with its own positioning.
• Risk isolation: sections with user-generated content whose quality cannot be controlled and which inflate crawl budget.
• Operational separation: a dedicated team, its own release cycle, separate hosting requirements.

The decision test is simple: if the content feeds the main site's topical authority, keep it in a subfolder; if it will live as an independent entity, a subdomain fits. Structural moves run through a 301 redirect map, and the effect is tracked against the pre-migration baseline; there is no universally right answer, only measurement.