Correct, the purpose of the dye is to identify the lymph node where the breast would 'drain to' in the event cancer cells infiltrated into the lymphatic system. The first node likely to be reached is usually (buy not always) also the closest node. The dye helps identify the actual 'first node ("sentinel") wherever it actually is. I strongly resisted the idea of having it done but changed my mind at the last minute. That node-negative biopsy was very valuable information that I almost didn't have...