If you are referring to a composite committee letter, then it is a composite of all your letters of recommendation (e.g.: a summary) which may or may not include tidbits from an interview by your pre-med committee.
For example, at my school, we need at least 3 LORs. We do an interview with one of the MD faculty, and they then write a committee letter that talks about you as a person (from the interview) and puts in the main points from the other LORs. They also attach your LORs to this committee letter, and thus acts kinda like a cover sheet.
Our school is slightly different.... just the pre-med advisor writes the summary based on at least four LORs and then sends copies of the originals too. So the composite letter seems a little unnecessary to me.
I had all the letters sent to my premed advisor. She composed a summary letter along with her thoughts as well. She then sent the original letters along with the summary.