The doctor takes the melanoma and it is looked at under the microscope. If all the edges are clear, then that means a successful removal. If some of the edges of the lesion are too close to normal skin, then he has to go back and make sure that an adequate margin has been achieved. On the one hand you don't want to remove more skin than necessary. On the other you want to be sure you got the whole lesion, so you rely on the microscope to tell you if you have it completely out.
Some cars go slow.Some cars race fast. If your melanoma is advanced, it means it can grow fast and it is more urgent to get complete 100% removal now.
Unfortunately once you have removed a melanoma from one area,it can pop up in another area. The key is frequent check ups and early removal.
Like many others,you can get confused by hearing all kinds of advice from dozens of sources. Skin specialists spend three years AFTER they become doctors specializing in skin disease. You MUST speak to your own skin doctor and fully understand what you have. Don't leave the office until you are sure you do understand,because frankly your life depends on it.