[ THERE IS NO MENTION OF PERCENTILE RANK IN THE CYCLE 4 PROPOSER'S GUIDE.  THEREFORE, I AM MARKING THIS ARTICLE AS DRAFT AND RETURNING TO THE CYCLE 3 CATEGORY.  WE CAN RE-PUBLISH AS NECESSARY - JAC ]

The assignment of proposal grades is described in Sec. 5.4 of the ALMA Users Policies and appendix B of the ALMA Cycle 3 Proposer's Guide and Capabilities. Specifically:

"Grade A is assigned to proposals solely on the basis of scientific rank. Grades B and C are assigned to proposals solely on the basis of scientific rank and Executive Balance. The maximum fraction of the available time that may be allocated to proposals of grades A, B, C is defined on a cycle-by-cycle basis, and announced in each Call for Proposals. "

In addition, starting in Cycle 3, "A" and "B" grades also consider the fraction of time allocated to proposals with "non-standard" components (which are restricted to take no more than 25% of the highest priority time), and "C" grades are also assigned on the basis of undersubscribed areas in observing parameter space (LST for a given configuration and band).

In addition to proposal grade, the email notification that is sent to PIs includes a ranking percentile range that reports the relative ranking of the proposal against the global list of projects.  Because of the other factors that affect grade assignment (Executive Balance, standard vs. non-standard, undersubscription), there is not always a linear relationship between the grade assignment and the proposal rank (particularly true of C graded proposals).