Different "category" for each subdomain?

My website has various subdomains, like
furniture.website.com
groceries.website.com
hardware.website.com
and I have a single Joomla! installation managing all subdomains.

Is it possible to configure a "category" for each subdomain, so that only the _relevant_ testimonials are displayed for each subdomain?

In the video I can see that a category would be specified for each review, but it is unclear to me what its function is.