After watching "The Omega Directive" episode of Voyager the other day, it got me thinking. Harry's initial designation was 6 of 10. When he "misbehaved", Seven redesignated him to 2 of 10, which he took as a demotion. Also, Seven's title was the Tertiary Adjuct of Unimatrix 01. This leads me to believe there's a Secondary that she "reports" to. Are there any other indications of Borg ranks, and how they get promoted?
Borg do not have "rank" per se -- this is evidenced in that same episode where, when Harry expresses surprise at the Borg having rank, Seven informs him that she learned it from her time on Voyager and is adopting it, the obvious implication being that it is not a normal part of the Borg Collective.
Borg do have functions, such as "Tertiary Adjunct of Unimatrix 01". This is not, however, a hierarchical designation1. Rather, she served as the third adjunct, similar to the arithmetic co-processors computers used to have back in the day -- Seven would have performed various tasks to facilitate the function of Unimatrix 01 (e.g. an arithmetic co-processor was used to perform floating-point arithmetic so that the primary CPU didn't need to).
The show never goes into detail on what precisely her function was, beyond being in love with the title; on more than one occasion, however, it is implied that she served as essentially another "co-processor", and this is backed up through multiple interactions with the Borg Queen where various drones are referred to as supplying "processing power". It's definitely not any form of "rank", however.
1 It is possible that "tertiary" referred to her "rank" in a sort of "line of succession" for the Adjuncts of Unimatrix 01, after Primary and Secondary. This is still not a "rank" in the same manner as we use one, it would merely have been a function that the Borg Collective assigned to Seven. That said, I believe "another adjunct", rather than "third backup adjunct", is more consistent with what's said and implied throughout the series.