Defense Custom Plays

What Does Base Form: Yes or No mean when creating custom plays for DEFENSE? How do I know which one to select?
Also, how does the defense perform when selecting different schemes? Is it safer just to keep it on BALANCED All the time? How does RUN vs PASS adjust preformance?
Also, how does the defense perform when selecting different schemes? Is it safer just to keep it on BALANCED All the time? How does RUN vs PASS adjust preformance?