Domain identifier only for one specific subdomain

In Swiftype site search, I am planning to use a domain identifier to filter results based on domain.
I've added 3 subdomains under one Swiftype engine.

Using the below approach I am planning to add domain identifier for one of the Subdomain, can I add it only for one subdomain or do I need to add it for all the Subdomains?

https://swiftype.com/documentation/site-search/guides/multiple-domains#filtering-domains

HI @iamvg , welcome to the Elasticsearch community!

Using the approach you mention, you just need to add meta tags to domains that you are going to filter on in your searches.

That is, if you only need search results from one subdomain, you only need to add meta tags to that domain.

In case you need to filter so that only results from other subdomains are displayed, you will need to add meta tags for those subdomains.

I hope that I have answered your question. Let me know about the result!