User blog comment:DisturbedDeaddMan/Spin-off Books Category/@comment-37434202-20190518071403/@comment-3452092-20190518151444

I would not call Rules of Engagement: Newlyweds" a spin-off. It's more of a finale. I'm honestly now sure I would call The Royal Heir as a spin-off of The Royal Romance either, it's more like when a series changes it's name mid run. Same series, different name. (Apparently this is what Pokemon has been doing for two decades and Power Rangers did this for a while, too. Power Rangers'' doesn't do it any more, but they did it in the past. . . I stopped watching when they stopped doing that.) As such I do not consider a direct continuation as a spin-off. A spin off can often have little in common other than being in the same world. As an example, here are some spin offs in the TV world that I watch or at least watched one of the shows involved:
 * NCIS is a spin-off of J*A*G*. Both NCIS: LA and NCIS: NO are spin-offs of NCIS.
 * The Flash (current version) is a spin-off of Arrow. Legends of Tomorrow is technically a spin-off of The Flash, although an argument could be made that it is a spin-off of both The Flash and Arrow. Batwoman is a spin off of Arrow, The Flash, and Supergirl. Not sure how to qualify Supergirl. I think it fits more in your sidequel definition.
 * CSI: Miami is a spin-off of CSI: Crime Scene Investigators. CSI: NY is a spin-off of CSI: Miami. I think CSI: Cyber was a spin off of CSI: Crime Scene Investigators.
 * Both Criminal Minds: Suspect Behavior and Criminal Minds: Beyond Borders were spin-offs of Criminal Minds.
 * A Different World was a spin-off of The Cosby Show
 * Fuller House is a spin-off of Full House
 * Xena: Warrior Princess and Young Hercules were spin-offs of Hercules: The Legendary Journeys
 * Highlander: The Animated Series was a spin-off of Highlander: The Series
 * Sam & Cat was a spin-off of iCarly and Victorious
 * VR Troopers and Masked Rider are some additional spin-offs of Power Rangers
 * Once Upon a Time in Wonderland was a spin-off of Once Upon a Time
 * So Random is considered a spin-off of Sonny With a Chance, but I'm not sure it's really a spin-off so much as a continuation without the titular character.

Examples from things I don't watch and don't know as clearly:
 * The Originals is a spin-off of The Vampire Diaries
 * The Legend of Korra is a spin-off of Avatar: The Last Airbender
 * A bunch of series were spin-offs of Happy Days including: Laverne & Shirley, Mork & Mindy, and Joanie Loves Chachi.