1. Terminology is remains from EXOS_User_Guide for older version of EXOS.
(if I'm not mistaken, hitless ugrade for X8 is able from EXOS 15.5, earlier versions support only for BD8000)
2. As BD can't work as cluster (as QFX), so I think hitless upgrade closelly to ISSU.
(I'm not so familiar with Juniper, so I can be wrong, may be colleagues correct me)
Thank you!