First of all, it would be useful to know what version of the model you are running.
If the most recent version ST-Sim 3.2.X this probably does relate to the “transition simulation group”. For earlier versions this would be the “primary transition group”. Targets should only be applied to either individual transition types or to “Transition Simulation Groups”. Individual transition types should only belong to at most one transition simulation group.
Did you get any warnings for your model run? You could see this by right clicking on the result scenario and selecting “Run Log”.