Ask yourself what one other possible piece of information might be helpful to your specific situation.
Haven't parented before? Have someone write a letter who has seen you with children -- a neighbor, a friend you've babysat for, a pastor in whose church you have taught Sunday school.
Have a lot of kids? Include something like 15 Reasons why Large Adoptive Families Make Great Resources for Waiting Children.
Looking to adopt a medically fragile child? Include a letter from a clinic or hospital where you have volunteered.
The idea with this collatoral information is that it shows people that you have thought things through. It can be as unique as you are.