A good structure would be:
Library Root
┖── Series Name
┠── Series Name v01.cbz
┠── Series Name v02.cbz
┠── Series Name v03.cbz
┖── Specials
┖── Series Name Omakes SP01.cbz
┖── Series Name 2
┠── Series Name 2 Vol.01 Ch.1.cbz
┠── Series Name 2 Vol.01 Ch.2.cbz
⋮
┖── Series Name 2 Vol.02 Ch.6.cbz
Kavita treats multiple types of files as "Specials" and will group them in a separate tab in series detail. Special marker (SP) will be removed from the UI title, however is used when ordering.
An entity is considered a special when:
A series can be parsed out of it, but no volume or chapter information is found.
Library Root
┖── Series Name
┖── Series Name.cbz
There are keywords in the filename like "Specials", "Omake" "OneShot", "Extra", "Art Collection", "Side Stories"
Library Root
┖── Series Name
┖── Specials
┖── Series Name Omakes SP01.cbz
To force a Special status, the filename can use SP01, SP02 etc.
Library Root
┖── Series Name
┠── Series Name v01.cbz
┠── Series Name v02.cbz
┠── Series Name SP01.cbz
┖── Specials
┖── Series Name SP01 Special Name.cbz
Specials will fallback to folder name for Series name. If you have a different name than the series files, then your special may not properly group.
└── Manga 2
└── Vol. 1
└── Ch.1 - Ch.3
├── 1.zip
├── 2.zip
└── 3.zip