Short answers, pulled from the story.
A bullion coin is struck from highly refined precious metal and held as a store of value or investment rather than used in everyday commerce. Unlike regular coins, bullion coins are distinguished by their stated weight and fineness, and carry a legal tender face value that is almost always lower than the actual market value of their metal content.
Bullion coins can range in fineness from 91.9% purity, corresponding to 22 karat gold, up to 99.99% purity at the 24 karat standard. Coins at 900 thousandths fine or above qualify as investment coins under United Kingdom rules.
Bullion coins sell at a premium over the commodities exchange price because of costs associated with manufacture, storage, and distribution, along with prevailing demand. Numismatic or collector value can also push the price above the metal's market value.
Investment coins are exempt from value-added tax on transactions in the United Kingdom. A coin qualifies for this exemption if it was minted after 1800, is at least 900 thousandths fine, has been legal tender in its country of origin, and is not normally sold at more than 180% of the value of its precious metal content.
Bullion coins are minted by government mints and carry a legal tender face value, while rounds are produced privately. Under United States law, pieces not stamped by authority of a government cannot be called coins and must instead be advertised as rounds.
The American Eagle and the Canadian Maple Leaf are the only two bullion coin series available in all four metals: gold, silver, platinum, and palladium.