What is the difference between "Embed fonts in the document" and "Only embed fonts that are used in documents"?

In File -> Properties -> FontEmbedding
there are 2 options

 Embed fonts in the document
 Only embed fonts that are used in documents

Embed fonts in the document is clear to me. But what does only embed fonts that are used in the documents mean? If you have used some fonts, then doesn’t the first selection cover your wish?

In the first selection, more fonts are embedded than used in the document.

Ok, but which ones? And why is the second selection referring to "documents? Are then fonts used in eg. doc2 also embedded into doc1?

At 1., as far as I know all the fonts of LIbreOffice.
At 2., only the fonts used in this document. So it only refers to the currently opened document.

You can do a test.
Use version 1 and export the document as PDF.
Then you do the test with version 2.
Then look at the file sizes of the two PDF files.

I suspect the word “fonts” should properly be called “font”.

I saved my document 3 times. I used some non common fonts.

Doc1: no options used: file size = 19,5 kB, saved as pdf, file size = 82,2 kB

Doc2: first option selected: file size = 76,3 MB, saved as pdf, file size = 76,4 MB

Doc3: second option used: file size = 19,5 kB !, saved as pdf, file size = 82,2 kB !

It seems that option 2 has no effect. It looks indeed that option 1 embeds all fonts.

Many PDF viewers have a Properties option which will tell you about the fonts in the document.

Actually, the option “Only embed fonts that are used in documents” is ignored unless you also check " Embed fonts in the document".
Embedding every font under the sun doesn’t make sense to me anyway, but if you want to share the fonts used to edit a document, you have to check both options.