Genres/ Subjects/ Categories in epub's metadata

What is the proper way to define genre in epub’s metadata? (in OEBPS/content.opf file.)

I can’t google the definition, just some obviousness mentions like "It may also include many other things such as … the genre" (link)

And, by the way, where can I find the current list of literary genres? (I found one in the wiki, but I’m not sure, is it applicable here or not.)

UPD 28.12.2019

I found in some downloaded epubs that genres are listed in several tags called subject from DDI-Codebook namespace, e.g.:

    <package xmlns:dc="http://purl.org/dc/elements/1.1/" ...>
    ...
    <dc:title>A Journey to the Centre of the Earth</dc:title>
    ...
    <dc:subject>Science fiction</dc:subject>
    <dc:subject>Adventure stories</dc:subject>
    <dc:subject>Earth (Planet) -- Core -- Fiction</dc:subject>
    <dc:subject>Voyages, Imaginary -- Fiction</dc:subject>

On the ddialliance.org I found a corresponding page with this obscure description:

Namespace: http://purl.org/dc/elements/1.1/

Type: SimpleLiteral, inherited from any

Content: mixed (allows character data), 1 attribute

Subst.Gr: may substitute for element any

Defined: globally in dc.xsd; see XML source

Used: never

Is it really correct to define genres in this tags? Or is it "de facto" standard without strict rule?