Mission President Recommendation Letter for BYU?

My Stake President told me that mission presidents can send out some sort of special recommendation letter for a student applying to BYU. As in, it's not really an endorsement, but a special recommendation. And usually he only sends out a few of these per year. Also, BYU would regard this as very important above all other critieria in making their decision to accept the student. Like it's similar to a Congressman writing a recommendation letter for someone trying to get in to a military school or something like that/ college..

He said it's some sort of secret... But, I just emailed my mission president (I'm an RM) and he seems to not know much about this..

Can anyone explain more about this? or if nobody has ever heard of it before?

I think the mission president would do the ecclesiastical endorsement if the missionary is going straight from his mission to BYU. Most people either apply before their mission so are considered continuing students, or they spend a month or two at home after their missions, making them able to get an ecclesiastical endorsement from their stake president. See this page:
https://it.byu.edu/byu/sc_help.do?syspa ... a069da1507