Subdomains Should Include Separate Robots.txt
Robots.txt is per hostname and protocol. If a page contains content from a different subdomain or domain, Google would respect the robots.txt for the main domain for the primary content. For the embedded content, Google would look at the robots.txt for the domain or subdomain that it is from and respect that.
You Can Publish Different Language Variations of Content Without Using Subdomains or Subdirectories
Content in different language variations should be on different URLs, but it doesn’t necessarily need to be put on subdirectories or subdomains. For example, this could be achieved with different parameters.
Having Separate Sections of a Site on Subdirectories Can Make Expansion Easier
If you are planning on expanding your site and adding additional sections to the site like a blog or ecommerce section, it can be easier from a maintenance perspective to have these on subdirectories.
The Site Diversity Update Won’t Affect How Subdomains Are Crawled
The new change that was launched to show more diversity of sites in the search results won’t impact the way subdomains are currently crawled and processed, it will only impact how they are shown in the search results.
International Websites on Separate Subdomains Will Not Be Penalized for Duplicate Content
Google will not penalize international websites that exist on separate subdomains if they have duplicate content. Instead, it will recognise the pages are identical and in most cases index both, but will only pick one URL to show in search results.
Putting Resources on a Separate Subdomain May Not Optimize Crawl Budget
Google can still recognise if subdomains are part of the same server and will therefore distribute crawl budget for the server as a whole as it is still having to process all of the requests. However, putting static resources on a CDN will balance crawling across the two sources independently.
Submit Reconsideration Requests Separately For Different Subdomains
Submit reconsideration requests separately if manual actions have been received for different subdomains, and make sure the disavow files are updated for each one.
Avoid Having Domains Additionally Accessible as CDN Subdomains
If the same content exists on a main domain and as a subdomain of a CDN, it can be indexed separately. This also means Google will have to crawl more to see the same amount of content. Use redirects, canonical tags, internal linking and sitemaps to set a preferred version.
Country-Specific Content Requires Different Domains, Subdomains or Subdirectories
For country targeting, Google requires a clear distinction between country-specific content by using separate domains, subdomains or subdirectories.