"letters of assurance" are rare this year, and apparently reserved for outreach minority applicants and athletic recruits. But if you did receive an LOA, and you have also received a nomination and you are medically qualified, then you will receive an appointment as indicated in the letter. However, most candidates this year received a "letter of encouragement" which does not set aside an appointment for the recipient, and so it is possible to receive an LOE and not receive an appointment (ie, if you are in a congressional district that uses a competitive slate, and someone else ends up with a higher WCS).