Core 1,2 and 3

I just uploaded my training plan, and at the end of the run workouts, it says that I can add Core 1, 2 or 3.  What are those?  I looked on the strength training drop down menu and there isn't anything labeled Core workout 1, 2 or 3.  Does it mean that I just need to pick 3 Core exercises and do them?  Or are there specific workouts listed somewhere?  (I combined the Half Marathon and the Half Iron training plans).

Thank you!!