Rochdale-Didsbury, Bury-Piccadilly, Bury-Altrincham etc. are the different routes, or at least that's what I think is simpler. The "Yellow Line" and the "Bury Line" are two different things after all.
Deansgate-Cornbrook, going off of my previous 'how the lines opened' logic would be on the Altrincham Line, since the Altrincham Line opened all at once between Deansgate and Altrincham, but again that's just what's simpler for me.
That is close to what Wikipedia seems to do with it. Most articles related to Metrolink just count Zone 1 as its own 'line' and leave it at that,
meaning that the Alty Line begins immediately south of Cornbrook (even before Cornbrook Junc), the Bury Line begins ~600m east of Victoria, and the East Manc Line begins somewhere on Merrill Street, but I think that doesn't make sense.
New Islington's article, for example, used to say it was on the 'Zone 1' line.
As I have said, I just count a line based on how it opened, meaning that the city centre is split into 1CC (Vic-Deansgate via Market St), 2CC (Lower Mosley St-Vic via Exchange Sq), and the spur to Piccadilly, which all opened separately from each other, and the E Manc Line begins just outside of Piccadilly's undercroft near Sheffield St.