[DrChiper]

Aha, so that's why I sometimes get the question to create a certain column because it is not present in the destination library, when I move ebooks between my libraries.
Until recently I never had paid much attention to the .OPF files and had just vaguely seen that a lot of quote's were used somehow:

Spoiler:

<meta name="calibre:user_metadata:#subtitle" content="{&quot;table&quot;: &quot;custom_column_6&quot;, &quot;column&quot;: &quot;value&quot;, &quot;datatype&quot;: &quot;text&quot;, &quot;is_multiple&quot;: null, &quot;kind&quot;: &quot;field&quot;, &quot;name&quot;: &quot;Subtitle&quot;, &quot;search_terms&quot;: [&quot;#subtitle&quot;], &quot;label&quot;: &quot;subtitle&quot;, &quot;colnum&quot;: 6, &quot;display&quot;: {&quot;use_decorations&quot;: false, &quot;description&quot;: &quot;Subtitle&quot;}, &quot;is_custom&quot;: true, &quot;is_category&quot;: true, &quot;link_column&quot;: &quot;value&quot;, &quot;category_sort&quot;: &quot;value&quot;, &quot;is_csp&quot;: false, &quot;is_editable&quot;: true, &quot;rec_index&quot;: 26, &quot;#value#&quot;: null, &quot;#extra#&quot;: null, &quot;is_multiple2&quot;: {}}"/>

Now that makes sense.
I did found too that I could remove that embedded metadata via the plugin "Modify ePub" whenever that might be needed. So everything is under control.