Adding this query string parameter will allow you to test a theme template on a page or product on a live site, without your customers seeing it.
If you already have a question mark in the URL, maybe for a variantId, then use an ampersand instead.
Where to find the “template-slug”
In your theme customiser at the top of the page, you can find the templates for different page types.
Then head to the page you want to test that template on and add the query string parameter to the address bar.
Note the page type needs to match, for example a collection template won’t apply to a product page type.
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