7B is the version of Darwin, then 85 are for Apple's use. So 85 means that there were 85 'released' (not to the world, but to QA) builds. Usually the build system bumps this value, so if there's a lot of work and quite a few releases to QA, there's no limit to the number after the Darwin version.
Once QA's happy with it, it's off to the CD or off to System Update with that build.
BTW: Builds labeled like this means that they can roll back to a specific version to do a patch for that version - like they're doing with the security updates.