@everything, the reason for the interest is that what is practiced under sub-lines of Yang style taichi can vary tremendously, to the point that they could be considered separate arts. Standing postures, spear work, Yang family baduanjin, the 2-man sanshou sets, etc - whether you will practice or have even heard of these depends a lot on which sub-line you are in.
Agree that talk of 'lines' doesn't make much sense when compared to standardised modern sports such as [sport] karate or judo....the more apt comparison is to Japanese koryu where they also discriminate between different lines of, say, aikijujutsu or kenjutsu, and for the same reason - the huge variance in curriculum.