This means that when working in an Interlaced format you are going to always perform the transition on Field 2... With set to Both then it will happen on which ever Field is next after the "cut" button.
Years and years ago there were edit systems that did everything on Field 2 and thus having the ability in a switcher to set to Field 2 was desired.
These kinds of restrictions don't really exist in modern Editing tools.
#Carbonite