If coin(s) were lost due to the loss of wallet keys or for some other reason. This loss is an indicative(estimated) market value when the coin(s) were lost. Logically the actual loss itself will need to be verified in some way.
On this screen it is possible to unclassify existing transactions; this will preserve the main information of the transaction and will allow you to reclassify it as a different type of transaction without needing to re-enter it. i.e. Lost -> Unclassified -> Make Purchase.
You can auto use the auto-value feature for looking up historic coin value.
For more information on auto-value click here.