If you do not have an LOA...and your MOC did not give a principal you compete for the 1 slot. They use your WCS to determine who gets charged to the MOC.
This is why they want you try for all of the sources you qualify for. You only need 1, but if you get 4 noms, than they can charge you to a different source...i.e. Sen A and Sen B you have the highest WCS, you get charged to Sen A. Next highest candidate gets charged to Sen B. Add in a third candidate who was highest after the two of you on the MOC list, they are now the highest and get charged to Cong. C. Thus, you may be number 3 or 4 on the list, but still get it b/c number 3 can be assigned to pres. which would make #4 on the list as the highest WCS.
I hope I explained it right...and finaly there also the national pool, so the theory is you only need 1 nom to get in, but the more you get the more the SA can use to select you. At least that is how this was explained to me
Also remember selection may occur at different times for different MOC's. Your congressman may send in their list in Jan and every candidates name is in. The SA can select for them, but they cannot selct if not every cadet on the list has not completed, then the SA must wait until the list is complete or after the deadline has passed.