Like about everything else... when it suits the Company, then it is "core" work, and of course, when it does not, then it isn't "core" work. If the Company decides "core" work to be loading planes, running connects, delivering locals, etc. then it does so provided the "core" work does not fall below an average number of daily mainline flights-- that "core" work belongs to a contractor. So much for "core" work.
On a side note, I am having a difficult time wrapping my head around these claims of an estimated $6,500 annual insurance increase by the LUS should JCBA adopt the LAA plan. Unless there are some plans which the LUS employee pays $0 presently (for which I do not know of any), I do not think it is possible to get to the $6,500 difference. As my family status changed a few years ago, I am not directly dealing with the expenses anymore, so I am not sure.
Also as I have some communications with a few preferential LUS hires, my understanding is the PTers on AA side pay the same as the AA FTers, which is less than the PTers on the US side who pay twice as much LUS FTers. (That statement reads like a college entrance exam question.) In essence, if my understanding to be correct, PTers on AA presently pay less than PTers on US, so they only group who will pay more would be LUS FTers with the company proposed JCBA?