Collectible coins include various types such as proof sets, which feature coins minted with higher quality finishes and details intended for collectors. The 2016 United States Proof Set includes a selection of coins with the "S" mint mark, showcasing enhanced characteristics due to a polished die process. This set typically features the 25-cent coin along with other denominations, making it a sought-after item for numismatists and coin enthusiasts interested in American currency.