Why ssl_trusted_certificate directive isn't defined in server block when I choose other type of certificate than Let's Encrypt?

[dominikkrulak]

Help request

Let's say I choose not to use Let's Encrypt certificate but any other paid certificate for domain.
When certification type Let's Encrypt is enabled I see ssl_trusted_certificate directive added.
When certification type custom certificate is enabled I see no ssl_trusted_certificate directive added.

Why is that?

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I try to respond to "Why is that?" question rhetorically.

Is this because you assume that root and Intermediate CA certificates chain is merged to /etc/nginx/ssl/example.com.crt in ssl_certificate directive?

Wouldn't be more elegant to define root and Intermediate CA certificates under ssl_trusted_certificate directive and server certificate under ssl_certificate?