When applied to vitimins what is a ug?

"ug" is the symbol for a microgram, which is one-millionth of a gram. There are 454 grams in a pound, if that helps. A milligram (symbol mg; one-thousanth of a gram, hence 1000 times more than a microgram) is enough of a substance that it can be clearly be seen by the naked eye as a small amount, and readily weighed in a good analytical lab balance. It is difficult to weigh a microgram, and requires specialized equipment.