It's not clear to me what this means.

I'll be honest, it isn't super clear to me either. @autoantwort can you clarify?

If we have for example a library A that can be build with openssl or boringssl the following would be tested by default:

1. a[core] "core"
2. a[core, openssl] "test-features"
3. a[core, boringssl] "test-features"
4. a[core, some-feature] "test-features"
5. a[core, openssl, boringssl, some-feature] "test-features-combined"

Now it is often the case that you can not select openssl and boringssl at the same time. If the lib works when no ssl backend is selected you would choose a[core, openssl, boringssl] = options which would result in

1. a[core]
2. a[core, openssl]
3. a[core, boringssl]
4. a[core, some-feature]

But in case you must at least select one ssl backend you would write a[openssl, boringssl] = options which would result in

1. a[core, openssl]
2. a[core, boringssl]
3. a[core, openssl, some-feature]

In the example block there is the following

## Examples:
##    qt3d[extras] = options # extras will be always added to the test combination (it is currently a requirement)
##    curl[core,openssl,wolfssl] = options # openssl and wolfssl are exclusive ssl backends but none must be selected
##    libgit2[pcre,pcre2] = options # you have to select pcre xor pcre2

I think "tests" is correct rather than "installed" as the intended outcome isn't an installation. Moreover, the following block "the following tests are performed" doesn't really make sense in an "installs" context.

I applied everything else.

Also needs --for-merge-with