Letters of recommendation - how to assign to schools in LSAC?

I know most schools accept 2-3 letters of recommendation. I've asked 4 contacts (2 academic and 2 professional) for letters and all have said yes.

Obviously I won't submit more than the maximum/recommended numbers of letters to each school, but I'm wondering how, in LSAC, you can assign specific letters to specific schools. Would that be through the "description" for each LOR?