Where does this comes from?

Discussion started on MangaManager and Kavita support channels

MangaManagerORG/Manga-Manager#185

What is the rationale for adding support for this element?

I have some series (mainly manga) that release booklets or additional content bundled with limited editions of a given volume. These are commonly referenced as VolNumber.5 as they are intended to be read after said volume.

Adding decimals to the volume tag would allow reader and organizing software to display these between vol n and vol n+1

Is the element already handled by any application or tool?

Not that I'm aware of