Well there isn't any as you've listed the same exact thing twice.:smileywink:
I think you are asking what's the difference between "sub summary by (leading)" and "sub summary by (trailing)"
A leading sub-summary will be placed above the body part and is created when you select the "Print Above" option when you first create the part. A trailing sub-summary prints below the body part and is created when you select the "Print Below" option.