Salvageable Wreckages are Signal Sources that can appear during supercruise at any point during your travels. They can contain Occupied Cryopods and other Canisters which are Illegal Salvage and must be sold at Black Markets. The main attraction to entering these Signal Sources is that they can also contain small salvaging vessels that are typically Adder size. The salvage within will never decay as abandoned or pirated Canisters will. And so you can take as long as you like if there is a large number. In this they are similar to Weak Signal Sources and are preferable to smugglers who don't enjoy pirating ships.