Hi, Turtletrekker. ZSG should actually be part of the 'primary core continuity' section because its final page blatantly states "The saga of Deep Space Nine will continue", a proclamation that is only included in novels which are part of the 'core' required reading list for the DS9-R.
I think a better reason would be that it's a Bashir story and he's a member of the core DS9 cast.
Maybe I'll just stick both ZSG
(With Bashir/Ezri) and RBoE
(With Sisko/Vaughn/Kira) in "Primary" and PoD
(with Shar) and IFM
(with Nog) in "Secondary".