under preparation of your NIW recomm letters (not signed yet), you need to put a more general words in "RE: field" of letters, such as "In Support of Dr. XXX's I-140 petition" if you plan to use same set of NIW letters for your EB1 application. But in your petition/cover letter, you must clearly indicate that what category you are seeking for.
If in case part or all your letters have been signed already, then just use it for EB1. For those not signed yet, do as I described above.
Just for your reference!