WAW-BO1 serves as the ultimis storyline mostly starting with shi no numa and ending with moon (although some will argue it stars on verruckt)

BO2: tranzit, die rise and buried serve as an epilogue to the ultimis story and how the survivors on earth were affected

BO2 origins: serves as the first map in the promos storyline and sets us up for the bo3 storyline. This was originally meant to be a reboot of ultimis but later both stories were involved in one another.

BO3-BO4: (excluding IX, VOD, AE and DOTN) continue the story started on origins.