Maestro is a term given to the most skilled and celebrated performers of Cybertron. While initially the term was exclusive to Harmonex (unsurprisingly), performers from across the planet eventually pushed for the ability to gain the esteemed title. Harmonex' leadership at the time felt that the increase in culture and creativity was bound to show the planet new and exciting forms of productions.
The Maestros are also given tiers within the overall grouping, Planetary, Solar, and Galactic levels, each with their own criteria of advancement, usually a peer review and addition to the craft, much in the same manner we equate to a Ph.D.
While there are dozens of those who ended up in the tiers, the Galactic tier was reserved solely for the three most noteworthy musicians on planet, with each one having a Solar level attendee that they have approved to take their place when then time came. This raised disputes within the community about how the Galactic tier were not necessarily the most skilled, but that they garnered the most favor with their predecessors. By the time of the Great War, Galactic tier (while still incredibly impressive) was considered more of a figurehead position within the ranks of the Maestros, a way to celebrate the memories of their predecessors, while First Sol and Second Sol were more hotly contested seats.
At the beginning of the Great War, the three seats of the Galactic tier were as follows
Maestro Primo - Vococent
Maestra Secunda - Imager
Maestra Trito - Repertoire