NSS and NBU are released together, so for instance NSS 8.0 was released at the same time as NBU 8.0. All NSS releases are backward compatible with NBU to version 7.61. However the NBU version cannot exceed the NSS version. So for instance using NSS 7.6.1 and NBU 7.7 is not supported. However you can use NSS 8.0 with any version of NBU between 7.6.1 and 8.0.
So generally if you had NSS 7.7.3 and NBU 7.7.3 and you want to upgrade these to the latest version, you should upgrade NSS first (it will handle older versions of NBU before you upgrade it), then upgrade NBU to the same release.
