[Q] Embedding metadata in EPUB3 ebook

[DrChiper]

@Kovid: Hi Kovid, just a question.
I have a few EPUB3 files in my repository and on inspection I noticed that the function "Embed metadata" embedded, among other things, "calibre:user_metadata" in the according .OPF file. The embedded entries seems to be restricted to the by myself defined extra columns.

Spoiler:

<metadata>
[. . .]
<opf:meta property="calibre:user_metadata">{
"#subtitle": {
"#extra#": null,
"#value#": null,
"category_sort": "value",
"colnum": 6,
"column": "value",
"datatype": "text",
"display": {
"description": "Subtitle",
"use_decorations": false
},
"is_category": true,
"is_csp": false,
"is_custom": true,
"is_editable": true,
"is_multiple": null,
"is_multiple2": {},
"kind": "field",
"label": "subtitle",
"link_column": "value",
"name": "Subtitle",
"rec_index": 26,
"search_terms": [
"#subtitle"
],
"table": "custom_column_1"
}
}</opf:meta>
</metadata>
<manifest>
[. . .]
</manifest>
<spine>
[. . .]
</spine>

Can you tell me what the idea behind this is (or was) as I do not believe it will ever be used by my e-reader.

[kovidgoyal]

All metadata is embedded in books. This true for epub 2, epub 3 and even pdf. Your reader wont use it, but if you ever need to re-import that book into calibre it will be used.

[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.