The Specific Lot accounting method is one way to calculate cost basis. Once you choose any accounting method you must continue to use the same method for the life of the associated investment.
When selling shares using the Specific Lot method you specifically choose which shares you sold. This method provides the most flexibility, but also requires you to manually select which shares were sold for each sell transaction. You do this using the Specific Lot ID Dialog.
To visually see which lots are available, see the "Un-specified Lot" markers.